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The effect of changes in atmospheric pressure on the occurrence of the spontaneous onset of labor in term pregnancies
The effect of changes in atmospheric pressure on the occurrence of the spontaneous onset of labor in term pregnancies Kenneth L. Noller, MD," Laurence J. Resseguie, PhDfl and Valerie Voss b Worcesleg, Massachusetts, and Atlanta, Georgia OBJECTIVE: Our purpose was to determine whether there is a relationship between changes in atmospheric pressure and spontaneous onset of labor in term pregnancy. STUDY DESIGN: All women admitted to Medical Center of Central Massachusetts-Memorial Hospital with spontaneous onset of labor at term and who were delivered on the service during a 12-month period represent the cohort for this study. Each maternal chart was abstracted to ensure that each member of the cohort met the inclusion criteria. Hourly recordings of atmospheric pressure made at the Worcester Station of the National Weather Service, Department of Commerce, were used as the meteorologic data points of interest. Least-squares regression was used to determine an equation that expresses the probability of the onset of labor in this cohort as a function of gestational age, which was used to calculate expected numbers for the statistical analyses. Two relationships were studied: (1) the ratio of the observed to the expected number of onsets of labor and (2) the initiation of labor and atmospheric pressure changes in the preceding 3 hours. RESULTS: Three-hour periods of falling atmospheric pressure were less often followed by initiation of labor than were the periods with other types of pressure sequences. No association was observed between the onset of labor and days of low mean pressure. CONCLUSION: Although there was an observed statistically significant association between falling barometric pressure and onset of labor, the magnitude of the difference is not of clinical significance. (AM J OBSTETGYNECOL1996; 174:1192-9.)
Key words: Barometric pressure, atmospheric pressure, weather, labor, onset of labor, pregnancy
During the past two decades there have been a n u m b e r of articles that have e x a m i n e d the effect of changes in weather on h u m a n parturition. The majority of these studies e x a m i n e d the relationship between declining barometric pressure and premature spontaneous rupture of the fetal membranes. ~-4 All the studies identified a statistically significant association between these two events. In the English literature only one recent study e x a m i n e d the relationship between the onset of uterine contractions and barometric pressure changes. Driscoll and Merker ' r e p o r t e d an association between the spontaneous onset of labor and those days when the barometric pressure was low. This association was statistically significant but was based on an assmnption that did not include daily fluctuations in the size of the population of pregFrom the Department of Obstetrics and Gynecology, University of Massachusetts Medical ,School," and the Cable News Network. b Presented by invitation at the Fourteenth Annual Meeting of the Anwrican Cqnecological and Obstetrical Sodety, Napa, California, September 7-9, 1995. Reprint requests: Kenneth L. NolleT; MD, Department of Obstetrics and (~necolog3, University of Massachusetts Medical Cent~ 119 Belmont St., Wcrrchest~ MA 01605. Copyright 9 1996 by Mosby-Year Book, Inc. 0002-9378/96 $5.00+ 0 6/6/71595 1192
nant womenf' O t h e r studies have investigated whether an association between changes in the weather and eclampsia or phases of the m o o n and either delivery or spontaneous onset of labor could be identified. 6' 7 N o n e of these three studies could find such associations. All of the studies, whether positive or negative, suffered from relatively small sample sizes and the use of statistical techniques that did not adequately address the many problems encountered with such research. In spite of the absence of meaningful scientific information linking the onset of uterine contractions to changes in atmospheric pressure, it has b e c o m e increasingly c o m m o n for labor and delivery physicians and nursing staffs to believe that weather changes can cause an increase in the n u m b e r of w o m e n admitted for labor. The purpose of this investigation was to d e t e r m i n e whether an association exists between the spontaneous onset of labor in term pregnancy and changes in atmospheric pressure. If such an association were identified it could be of more than anecdotal significance. In this era of strict cost control in medicine in the United States, labor and delivery nursing staffs and other support personnel have often been downsized dramatically. The ability to predict, with some reliability, the n e e d for additional nursing staff
Volume 174, Number 4 AmJ Obstet Gynecol
some hours in the future has the potential to be of great clinical and financial benefit. Material and m e t h o d s
All deliveries occurring on the obstetric service of the Medical Center of Central Massachusetts-Memorial Hospital from Oct. 1, 1993, through Oct. 31, 1994, represent the case material for this study. Although the duration of interest was the first 12 months of the period, deliveries during the thirteenth month were abstracted to provide information for the probability calculation that was necessary for the statistical analyses (see below). A total of 3813 deliveries occurred during the 12-month period, Oct. 1, 1993, to Sept. 30, 1994. Because a copy of the maternal labor and delivery record is inserted into each newborn's chart, it was possible to obtain information from this source in those few cases where the maternal record was not available. Maternal age, last menstrual period, estimated date of confinement (EDC), date of birth, gravidity, parity, date and time of onset of contractions, date and time of spontaneous rupture of membranes, date and time of delivery, sex of infant, type of delivery, and all obstetric interventions were abstracted from the original records. In most cases the information was abstracted from the nurses' labor and delivery notes. When information was missing from this source, the senior resident admission note, the j u n i o r resident history and physical examination, the attending physician's notes, and other miscellaneous information found in the chart were abstracted, in that order. Maternal transports were culled from the data set because they represented women from practices other than those at Medical Center of Central Massachusetts-Memorial Hospital. In addition, the amount of available data concerning the EDC was variable, and most were not eligible because of preterm onset of labor or use of tocolyric agents. In addition, although most maternal transports still resided within a 50-mile radius of Medical Center of Central Massachusetts-Memorial Hospital, in general they lived farther away from the obstetric service and thus might be exposed to different climatic conditions than those women that comprise the "natural" delivery population. Changes in atmospheric conditions are recorded hourly by the National Weather Service located at the Worcester Municipal Airport. Copies of the daily logs of these readings were obtained for the study period. In addition, these data were obtained from the National Oceanographic and Atmospheric Agency, National Climatic Data Center, Asheville, N.C., in computer-readable format. The daily logs were used to supply data points missing from the computer tape. Barometric pressure recordings were taken and recorded hourly and represent the sole atmospheric measure used in this study.
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Any attempt to detect an effect of atmospheric pressure on the initiation of labor requires some method of specifying the expected n u m b e r of initiations of labor in the absence of the weather influence. This study was restricted to women who had spontaneous onset of labor after completion of 37 weeks of gestation and before completion of 42 weeks of gestation. Therefore all women who had onset of labor between 37 and 42 weeks' gestation between Oct. 1, 1993, through Sept. 30, 1994, compose the cohort of interest. However, for calculation of the expected n u m b e r of onsets of labor it was necessary to include those women who began labor during the first 13 days after the end of the study year. For each of the 365 days of the study period, the gestational age was determined for each of the study women who potentially could have began term labor on that date. The expected n u m b e r of initiations can be determined by the formula
~12=_21P,N, where tis the gestational age relative to the EDC (t_21 = 37 weeks, t+l.~= 41 weeks 6 days), Ptis the calculated probability of initiation of labor on gestational day t, and N t is the number of women in the pool with gestational age t on that date. Least-squares regression was used to develop an equation that expresses the probability of initiation of labor for any women in our study cohort for any given gestational age. The probability model was then used to calculate the expected n u m b e r of onsets of labor that would be expected to occur on each day during the study year. The mean barometric pressure for each calendar day was calculated, and the onsets of labor occurring during the lowest tercile of pressure were compared with those occurring in the other two terciles. Ranking of mean barometric pressure by terciles was used to allow direct comparison of our findings to those of Driscoll and Merker. 5 The t test was performed to compare the square root of the ratio of observed to expected daily events in the lowest 122 days to the remaining 243 days. The square root transformation of the data was necessary to achieve a normal distribution, which is necessary for the t test. The Wilcoxon rank-sums test was also applied to these data. The atmospheric pressure reading for each of the 8760 hours of the year was categorized by the changes in atmospheric pressure over the preceding 3 hours. The prior 3 hours were stratified as showing a continuous change in an upward direction, a continuous change in a downward direction, or some other combination. The ~2 statistic was used to test the hypothesis that the onset of labor was not associated with 3 consecutive hours of falling barometric pressure. A similar but separate test was used for the hypothesis that labor was not associated with consecutively rising barometric pressure.
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Table I. Deliveries at Medical C e n t e r of Central M a s s a c h u s e t t s - M e m o r i a l Hospital, Oct. 1, 1993, to Oct. 31, 1994
Remaining sample Total deliveries abstracted Failure to meet eligibility criteria* Labor onset not in 12month study period Hour of labor onset not known
1495
4142 2647
212
2435
31
2404
*Cesarean section or induction before labor, use of tocolysis, maternal transport.
All statistical analyses were p e r f o r m e d with SAS software package (SAS Institute, Cary, N.C.) for p e r s o n a l computers.
Results Table I shows the total n u m b e r o f deliveries o c c u r r i n g d u r i n g the study p e r i o d a n d the n u m b e r s a n d reasons for exclusion. T h e h o u r of o n s e t o f labor was k n o w n for 2404 deliveries that m e t the inclusion criteria for t e r m spontan e o u s o n s e t o f labor for the 3-hour pressure c h a n g e analysis. A total of 2435 w o m e n were available for the analysis of daily b a r o m e t r i c pressure because 31 additional w o m e n h a d i n f o r m a t i o n c o n c e r n i n g the day of o n s e t b u t lacked h o u r i n f o r m a t i o n . T h e m e a n n u m b e r of l a b o r onsets p e r day was 6.67, with a r a n g e of 1 to 16. Least-squares regression was used to develop a p r o b ability e q u a t i o n that c o u l d b e used to d e t e r m i n e a n exp e c t e d n u m b e r of l a b o r o c c u r r e n c e s d u r i n g e a c h of the 365 days of the o b s e r v a t i o n period. T h e e q u a t i o n that best d e s c r i b e d the probability for the 365-day o b s e r v a t i o n p e r i o d was
1 n(p) = -2.208542 + 0.128741 [CA] - 0.000381 [GA] 2 w h e r e [GA] is the gestational age in days, with gestational age 0 days at EDC. With this f o r m u l a t h e r a n g e of exp e c t e d initiations o f l a b o r p e r day was 3.94 to 9.57. Alt h o u g h t h e r e a p p e a r s to b e large variability in the o b served n u m b e r s of l a b o r onsets c o m p a r e d with the n u m b e r e x p e c t e d by the probability model, this difference is n o t statistically significant (p = 0.84). T h e fit b e t w e e n the probability m o d e l a n d the observed probability o f a s p o n t a n e o u s o n s e t of labor is shown graphically in Fig. 1. T h e only m a j o r deviation b e t w e e n the m o d e l a n d the o b s e r v e d occurs at 41 weeks 6 days of gestation, a n e x p e c t e d f i n d i n g because, by definition, all w o m e n in the study c o h o r t m u s t have h a d o n s e t of l a b o r by the last day, whereas in the probability' m o d e l it was possible for p r e g n a n c y to c o n t i n u e b e y o n d 42 weeks. Table I1 shows the results of the analysis of the lowest tercile of average a t m o s p h e r i c pressure c o m p a r e d with the o t h e r two terciles. O n the 122 days with the lowest average b a r o m e t r i c pressure, the probability of a sponta-
n e o u s o n s e t of labor was n o t statistically significantly dit: f e r e n t from days with h i g h e r pressure. This conclusion was similar w h e t h e r the t test or the Wilcoxon two-sample test was applied. However, the sample size (122 : 243 days) was n o t sufficiently large to detect a difference less t h a n ~/Observed Expected
0.057
with a power of 80%, with ~ = 0.05. We also e x a m i n e d the m o s t e x t r e m e low-pressure days by p l o t t i n g the ratio of o b s e r v e d to e x p e c t e d to the m e a n b a r o m e t r i c pressure; we f o u n d n o excess onsets of labor. Similarly, w h e n we fitted quadratic or cubic equations to the data in a n a t t e m p t to d e t e c t a n o n l i n e a r trend, n o n e was identified. Table III shows the o c c u r r e n c e of s p o n t a n e o u s onset of l a b o r o n the basis of 3-hour periods of pressure change. T h e r e was a statistically significant deficit in the occurr e n c e of s p o n t a n e o u s onsets of labor in the h o u r after c o n t i n u o u s 3-hour pressure decreases. However, n o association b e t w e e n the s p o n t a n e o u s o n s e t of labor at t e r m a n d increasing 3-hour b a r o m e t r i c pressure was observed. Sample size calculations were p e r f o r m e d to d e t e r m i n e w h e t h e r this study h a d sufficient power to detect such an association if it existed. Because of the large n u m b e r of observations in the study, t h e r e was sufficient power to d e t e c t a difference b e t w e e n o b s e r v e d a n d e x p e c t e d p r o b abilities for these variables at the 0c = 0.05 level with 80% certainty, with a difference between o b s e r v e d a n d exp e c t e d p e r c e n t a g e s as small as 0.5.
Comment O u r data failed to d e m o n s t r a t e an increase in the occ u r r e n c e of s p o n t a n e o u s o n s e t o f labor with falling barometric pressure. This is in c o n t r a s t to the c o m m o n l y held b e l i e f t h a t a strong a t m o s p h e r i c pressure d r o p causes labor to begin. O u r findings also are in c o n t r a s t to those of Driscoll a n d Merker, 5 w h o identified a statistically significant excess in t h e o n s e t of labor d u r i n g those days in which the m e a n b a r o m e t r i c pressure was low (lowest tercile). However, t h e r e a p p e a r to b e limitations to t h e i r study. First, they did n o t a c c o u n t for the daily variation in the size of t h e i r p r e g n a n t p o p u l a t i o n . Because the n u m b e r of w o m e n who are delivered o n a given day affects the n u m b e r o f w o m e n eligible for delivery o n the n e x t ( a n d every s u b s e q u e n t ) day, this seems to b e a deficiency of t h e i r analysis. We p r e f e r r e d to c o m p a r e actual labor onsets to onsets e x p e c t e d f r o m e n u m e r a t i o n of the specific p o p u l a t i o n of p r e g n a n t w o m e n p r e s e n t o n each day. Seco n d , it is n o t possible to d e t e r m i n e the n u m b e r of actual observations o n which the study of Driscoll a n d M e r k e r was made. T h a t is, it is n o t possible to d e t e r m i n e how m a n y w o m e n went into l a b o r d u r i n g the 2 4 - m o n t h period o f t h e i r study. Third, because the n u m b e r of w o m e n delivered o n t h e i r obstetric service was increasing d u r i n g
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Fig. 1. In (probability) of labor onset on a given day versus gestational age in weeks, where plus sign is observed probability of onset and solid line is derived probability function (see text).
T a b l e II. S p o n t a n e o u s o n s e t o f labor: Lowest m e a n a t m o s p h e r i c p r e s s u r e tercile (days) c o m p a r e d with o t h e r terciles
Days (No.)
Mean SQRTRAT
Lowest tercile
122
0.965985
Other
243
0.990066
t test* p = 0.26
Wilcoxon (rank sum) test? p = 0.37
SQRTRAT, Square root of ratio of observed to expected number of labor onsets. *Test of significance of difference between means, t = 1.135, 363 degrees of freedom. ]-Wilcoxon (rank-sums) test, S= 21473.0.
t h e study p e r i o d , the a u t h o r s a p p a r e n t l y " a d j u s t e d " t h e n u m b e r o f w o m e n b e i n g d e l i v e r e d u p w a r d to d e v e l o p a l i n e a r t r e n d for t h e i r p r o b a b i l i t y calculations. Because o t h e r a u t h o r s h a d d e m o n s t r a t e d b o t h a n increase in s p o n t a n e o u s r u p t u r e o f m e m b r a n e s a n d o n s e t o f l a b o r to b e associated with a t m o s p h e r i c p r e s s u r e decreases, we were s u r p r i s e d to find a significant d e c r e a s e in t h e o n s e t o f l a b o r after 3 - h o u r intervals w h e n a t m o s p h e r i c p r e s s u r e was d r o p p i n g . However, b e c a u s e o f the
T a b l e III. S p o n t a n e o u s o n s e t o f l a b o r in h o u r after 3 - h o u r interval with c o n t i n u o u s l y increasing, decreasing, o r o t h e r a t m o s p h e r i c p r e s s u r e c h a n g e s
Pressure change
No. of 3 hr intervals
No. of 3 hr intervals
Down Up Other
1809 (20.7%) 1799 (20.5%) 5152 (58.8%)
440 (18.3%)* 493 (20.5%)]1471 (61.2%)
*Continuously down pressure versus all others: Z2= 8.0872, 1 degree of freedom; p < 0.005. ]-Continuously up pressure versus all others: ;(2 = 0.0012, 1 degree of freedom; p > 0.97. relatively large n u m b e r o f o b s e r v a t i o n s (labor onsets) in o u r study, t h e c o n c e p t o f "statistical significance" is perh a p s misleading. In large studies even a m i n o r deviation f r o m e x p e c t e d can result in a small p value. N o n e t h e l e s s , we o b s e r v e d a deficiency in the o n s e t o f l a b o r >10% of the expected. T h e o n s e t o f l a b o r is k n o w n to vary in a d i u r n a l m a n ner.8. 9 In these studies t h e p e a k o n s e t o f l a b o r o c c u r r e d b e t w e e n m i d n i g h t a n d 2 AM. W h e n we e x a m i n e d o u r data for t h e time o c c u r r e n c e o f l a b o r onset, we also o b s e r v e d t h a t m o s t of t h e labors b e g a n b e t w e e n m i d n i g h t a n d 4 AM.
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April 1996 ,~nJ Obstet Gynecol
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Fig. 2. Percent versus h o u r of day where plus sign is percentage of days with 3-hour continuously dropping pressure ending at h o u r of day a n d circle is percentage of onsets of labor during h o u r (n = 2404).
When we c o m p a r e d the fraction of labor onsets that occurred at any given h o u r with the barometric pressure change for the preceding 3 hours for the same h o u r of the day, we found an interesting association (Fig. 2). As can be seen from this figure, the peak occurrence of decreased barometric pressure roughly coincides with the nadir of labor onset. W h e t h e r this is a chance association, an association resulting from a third factor (e.g., absence of light), or whether there is a cause-and-effect relationship is impossible to say. However, a considerable n u m b e r of physiologic changes do occur on the basis of circadian rhythm, and uterine activity should not be excluded from consideration. O n e of the reasons this study used the time period Oct. 1, 1993, to Sept. 30, 1994, is the fact that it coincided with one of Worcester County's most meteorologically unstable years. The winter was plagued by several large snow storms (Nor'easters) with a total accumulation of snow to a depth of nearly 10 feet. We hypothesized that, if the onset of labor were associated with the passage of a "front," the selected time period might have identified such an association. Unfortunately, although frontal theory remains an integral part of meteorologic thought, this theory, first de-
veloped by a group of Norwegian scientists during World War I, defies explicit definition. Fronts are generally defined as the boundaries separating air masses of different densities. Such definition implies a temperature contrast, atmospheric pressure troughs, changes in wind direction, moisture discontinuity, and certain characteristic cloud and precipitation forms. T h e r e are warm fronts, cold fronts, and stationary fronts. Some fronts are associated with a rapid drop in air pressure, but there are times when a weather front passes without appreciable change in atmospheric pressure. O n the other hand, some s t o r m s - - s u c h as the classic New England " N o r ' e a s t e r " can induce an intense drop in pressure without the observation of a frontal passage as defined by a meteorologist. Thus there were no combinations of information collected from the Worcester Weather Station that could be used to write a program that would identify a weather "front." Indeed, meteorologists identify weather fronts by looking at the current and previous recordings at their weather station and they identify a front and its passage more by experience than by definition. Although this is the largest and most exhaustively analyzed study of the relationship between the spontaneous onset of term labor and atmospheric pressure, our restdts
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do not absolutely p r e c l u d e the possibility that pressure changes also can be positively associated with the onset of labor in o t h e r geographic areas. O u r data are reliable only for o u r specific hospital, for a specific year, with the practice patterns in place at the time of the study. REFERENCES
1. Polansky GH, Varner MW, O'Gorman T. Premature rupture of the membranes and barometric pressure changes. J Reprod Med 1985;30:189-91. 2. Milingos S, Messinis I, Diakomanolis D, Aravantinos D, Kaskarelis D. Influence of meteorological factors on premature rupture of fetal membranes. Lancet 1978;2:435. 3. Steinman G, Kleiner G. Spontaneous premature rupture of membranes: correlation with ambient atmospheric pressure. N Y State J Med 1978;78:900-1. 4. Boland N, Byers R, Kitay D. Meteorologic phenomena and labor initiation. Atlanta (GA): Department of Gynecology and Obstetrics, Emory University School of Medicine, 1980: 168. Bulletin No. 2. 5. Driscoll D, Merker D. A search for associations between weather and the onset of human parturition. IntJ Biometeorol 1984;28:211-24. 6. Alderman BW, Boyko EJ, Loy GL, Jones RH, Keane EM, DalingJR. Weather and o c c u r r e n c e of edampsia. IntJ Epidemiol 1988;17:582-8. 7. Witter E The influence of the moon on deliveries. A_,~J OBs'rEz GWECOL1983;124:637-9. 8. Cooperstock M, England J, Wolfe R. Circadian incidence of labor onset hour in preterm birth and chorioamnionitis. Obstet Gynecol I987;70:852-5. 9. Fraser W, McLean F, Usher R. Diurnal variation in admission to hospital of women in labour. CanJ Surg 1989;32:33-5.
Discussion
DR. ROBERT C. CEFALO, Chapel Hill, North Carolina. Centuries ago the a p p o i n t e d time of labor was declared to be a law of God; many hypotheses have since a t t e m p t e d e n l i g h t e n m e n t , only to be displaced in time by others. We now know that at a species-specific time in gestation the growing fetus decreases its d e p e n d e n c e on the nutritional, respiratory, excretory, and e n d o c r i n e functions of the placenta, and parturition ensues. In most m a m m a l i a n species the multifactorial contributions of the fetus-placenta and m o t h e r to the onset of labor appear to be interrelated. Transient increases in fetal adrenal cortical secretion, changes in placental horm o n e p r o d u c t i o n relating to the progesterone and estrogen ratio, release o f phospholipase and arachidonic acid by the placental m e m b r a n e s , increased synthesis and release of relaxin and prostaglandins by the uterine epithelium, and increased myometrial sensitivity to ecbolic agents such as oxytocin and prostaglandin F2,~ have all b e e n implicated in the c o n u n d r u m of the onset of labor. T h e r e appears to be a disproportionate n u m b e r of events in the fetus that occur just before labor with only m i n o r maternal influences. 1 T h e m o t h e r ' s control is similar to her power over onset of puberty, adolescent impulses, or selection of a mate. But there are maternal factors: mares foal in darkness, rats deliver in daylight, and rabbits will not deliver u n d e r direct observation. T h e alpaca, an animal that resides high in the Andes, delivers between 5 ~\~ and 2 PM to take advantage of the sunshine; there are no births on very cold days. l' 2 In humans, controversy surrounds the question of
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whether spontaneous labor, p r e m a t u r e r u p t u r e of the membranes, or birth has a 24-hour rhythmicity and, if so, what role e x o g e n o u s factors may contribute. 3" ~ For many years there has b e e n an attempt to link m e t e o r o l o g i c factors such as weather, barometric pressure change, seasonal changes, tides, and m o o n phases to the onset of p r e m a t u r e r u p t u r e of membranes, p r e t e r m labor, labor, and h i g h e r prevalence of preeclampsia-eclampsia. Hippocrates wrote, " T h e changes of seasons mostly eng e n d e r diseases, and in the same season heat or cold could create great changes. ''~ In 1756 Smellie observed a clustering of cases of eclampsia and attributed them to changes in the weather. Many o t h e r writers have shared his opinion. In 1984 Chesley 6 f o u n d that the incidence of preeclampsia reached a peak in March and April; the meteorologists cited unsettled weather as the cause, whereas others believed that an increased n u m b e r of brides the previous J u n e had provided a surge of primigravidas. Anecdotal observations have b e e n m a d e linking the onset of labor and " b a d weather" conditions. Because the a p p r o a c h o f " b a d weather" is often a c c o m p a n i e d by a drop in atmospheric pressure, several authors have studied but not f o u n d a consistent relationship to changes in barometric pressure and the onset of labor. However, some authors cited by Noller et al. 5' 7 did find that a lower m e a n barometric pressure may be related to a h i g h e r prevalence of p r e m a t u r e r u p t u r e of the membranes. In addition to barometric pressure, o t h e r e n v i r o n m e n tal considerations may m o d u l a t e the process of parturition, such as variability in geographic latitude, climate, temperature, length of light-dark cycles, light intensity, and work schedules. T h e presence of the full m o o n has been ascribed powers to foster mental illness, crime, or an increase in the n u m b e r of births; fortunately, these ideas have b e e n misbegotten and unproved. However, there is a synchronicity of lunar phases with the rise and fall of the ocean tides along with the m o o n ' s ability to influence the weather through the creation of daily changes in the air pressure; this, in part, explains the coincidence of lunar phases with cloudiness, rainfall, and hurricane formation. T h e full m o o n may n o t affect your sanity, but it apparently unbalances the weather, in turn, the weather may be associated with barometric pressure changes that may be associated with the onset of l a b o r - but b e h o l d the study by Noller et al. T h r o u g h a sophisticated statistical analysis of the data, the authors have shaken my belief that the onset of labor is associated with falling barometric pressures and that the full m o o n brings on m o r e deliveries than o t h e r lunar phases. However, they c a n n o t take away the fact that blackouts, blizzards, typhoons, hurricanes, and o t h e r weather extremes do lead to an increased n u m b e r of deliveries approximately 40 weeks later! I must now ascribe my labor deck experiences not to fact but to bad luck. Howevel, as sophisticated the study by Noller et al. was, I have several questions. 1. O u r definition of clinical labor is the time of regutar contractions leading to the birth of a child. How do the authors account for the changes in the cervix and lower uterine s e g m e n t that have b e e n associated with regional uterine activity? How would the authors relate
1198 Noller,Resseguie, and Voss
.\p, il 1996 \ m J ()bSlet (;vnecol
these c h a n g e s to o n s e t of labor a n d b a r o m e t r i c pressure readings? 2. T h e a u t h o r s readily a d m i t that o t h e r e n v i r o n m e n t a l c o n s i d e r a t i o n s may m o d u l a t e the process of parturition, such as variability in g e o g r a p h i c latitude, prior p h o t o p e riod e x p e r i e n c e , light intensity, a n d work schedules. How d o the a u t h o r s a c c o u n t for these potential variables? 3. T h e a u t h o r s fail to m e n t i o n how the EDC was determ i n e d . How would a n inaccuracy affect their resuhs? 4. Is the fluctuation of b a r o m e t r i c pressure within a day (e.g., day vs night) larger t h a n fluctuation a m o n g days? Averaging b a r o m e t r i c pressure for each day may to some e x t e n t dilute the effects of low pressure. If this is true, t h e n prior 3-hour c h a n g e s in pressure m i g h t be a m o r e sensitive m e a s u r e m e n t t h a n a m e a n pressure for the day.
REFERENCES
1. I,iggins GC. Initiation of parturition. Br Med Bull 1979;35: 145-50. 2. Nathanielsz PW. The tketus and birth. Amsterdam: Elsevier, 1977:151-2. (Ciba Foundation no. 47.) 3. Ixmgo LD, Yellon SM. Biological timekeeping during pregnancy and the circadian rhythms in parturition. In: Kunzel W, Jensen A, eds. The endocrine control of the fetus. Berlin: Springer-Verlag, 1988:17.9,-.92. 4. Honnebrew MBOM, Nathanielsz PW. Primate parturition and the role of maternal circadian system. Eur J Obstet Gynaecol Reprod Biol 1994;55:193-203. 5. Bider"O, Sivan E, Seidman DS, et al. Meteorological factors in hypertensive disorders, vaginal bleeding and premature rupture of the membranes during pregnancy. Gynecol Obstet Invest 1991 ;32:88-90. 6. Chesley LC. History of the epidemiology of preeclampsia and eclampsia. Clin Obstet Gynecol 1984;27:801-20. 7. Steinman G, Kleiner G. Spontaneous rupture of" membranes--cnrrelation with ambient atmospheric pressure. N Y State.] Med 1978;5:900-1.
De,. MICHAEL C,. Ross, Los Angeles, California. H e a t stress has b e e n associated with catecholatnine, vasopressin, a n d oxytocin release, in a d d i t i o n to plasma volu m e contraction. Because m e t e o r o l o g i c data were available, did you control for the m e a n daily t e m p e r a t u r e ? DR. THOMAS H. KIaSCn~AUM, Bronx, New York. T h e r e is an implicit a s s u m p t i o n in the quadratic probabilio, function that you d e f i n e d t h a t the o n s e t of l a b o r follows b a r o m e t r i c c h a n g e s within 24 hours, a n d the t r e n d analysis makes the a s s u m p t i o n that t h e r e ' s little delay in the o n s e t of labor. You m i g h t c o n s i d e r looking at the possibility, that l o n g e r intervals or s h o r t e r intervals between b a r o m e t r i c events a n d o n s e t of labor may show something. DR. J o n s T. R ~ I ~ , Boston, Massachusetts. I can r e m e m b e r f i n d i n g an old article from the mid-1960s in which C a v a n a u g h suggested that the i n c i d e n c e o f p r e e c l a m p s i a a n d eclampsia in south Florida m i g h t c h a n g e d u r i n g h u r r i c a n e season. O n e of the issues I raised with a m e t e o rologist friend of m i n e at the time was how m u c h variabilitT is t h e r e in b a r o m e t r i c pressure from o n e location to a n o t h e r . W h a t is the proximity of the l a b o r a n d delivery u n i t a n d even the location of the patients' h o m e s to the Worcester airport, a n d how do you really know that your o b s e r v a t i o n is a reflection of the b a r o m e t r i c pressure t h a t
you were b e i n g given f i o m the m o r e r e m o t e location in Worcester? De,. MmnaEl. L. SOCOL, Chicago, Illinois. Because a significant p e r c e n t a g e of patieuts at t e r m have r u p t u r e d m e m b r a n e s b u t are n o t in labor, I'm interested to know how yon dealt with those patients in your analysis. 1 know a n u m b e r of patients were excluded, b n t it was unclear how many h a d s p o n t a n e o u s r u p t u r e of m e m b r a n e s n n t in labor. DR. NOLLER (Closing). Dr. Cefhlo asked how we acc o u n t e d for p r e l a b o r c h a n g e s of the cervix in o u r analysis. T h e r e ' s really, n o g o o d answer to that. We certainly c a n n o t postulate that w e a t h e r c h a n g e s a c c o u n t for the p r e t e r m c h a n g e s in cervical dilatation a n d e f f a c e m e n t that so m a n y w o m e n u n d e r g o . We suggest only that the trigger for the onset of labor, whatever t h a t is, m i g h t be i n f l u e n c e d in some way by c h a n g e s in the weather. Yon also asked a b o u t the o t h e r things, such as work, light, a n d latitude, that can affect the onset of labor. In o u r full article we discuss h o u r s of the day; t h e r e deftnitely is cyclicity. N i n e t e e n different studies have shown that the most f r e q u e n t time of the onset of labor is belween m i d n i g h t a n d 4 .~M, with a real p r e f e r e n c e for m i d n i g h t to 2 xx~. We also identified t h a t o c c u r r e n c e in o u r data. T h e r e are two answers to the question a b o u t o t h e r variables. O n e is pragmatic a n d o n e is statistical. Pragmatically, we c a n ' t a c c o u n t t o t all the variables. We can only a c c o u n t for those we t h o u g h t about, a n d I would suggest that n o one, in any study, can a c c o u n t for all the variables. T h e statisticians, of course, have a second answer to this question. They include all the o t h e r variables that nobody's t h o u g h t of in the " e r r o r variance." T h e waywe calculated the EDC was as I m e n t i o n e d . Is it accurate? Certainly it is not. In fact, I t h i n k it can be a r g u e d t h a t t h e r e is n e v e r a n accurate EDC. W h a t we h o p e is that, because we used a consistent m e t h o d , the errors in the calculation of the EDC were r a n d o m r a t h e r than systematic a n d t h e r e f o r e t h e r e s h o u l d n o t be any systematic bias i n t r o d u c e d into the results. In answer to the n e x t question, b a r o m e t r i c pressure did vary within days a n d f i o m day to day d u r i n g the year, T h e longest variation within any single day was similar to the m a x i m u m c h a n g e t h r o u g h o u t the year. T h e c o m m e n t t h a t using the m e a n daily b a r o m e t r i c pressure is an insensitive m e a s u r e is certainly true. W h e n we used the m e a n daily b a r o m e t r i c pressure, we f o u n d a small decrease in the o b s e r v e d versus the e x p e c t e d o n the low pressure days, b u t it wasn't significant. It was only w h e n we looked at 3-hour intervals t h a t we t o u n d a significant difference. 1 would agree that m e a n daily barometric pressure is insensitive. H e a t stress a n d t e m p e r a t m e c h a n g e s could be implicated. We have n o t looked into this. It's o n e of the bits of i n f o r m a t i o n we have f r o m the w e a t h e r service, a n d it is o n e of the things we h o p e to look at in the future. We did look at l o n g e r a n d s h o r t e r intervals, such as those in the U n i v e r s i ~ of Iowa study. They l o o k e d at 3, 6, 9, a n d 12 hours. We also l o o k e d at those, b n t because the 3-hour intervals were the most significant in t h e i r study it
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was the one that meant the most to us. We postulated that if weather was going to have an effect, it would be shortly before the onset of labor. One of the reasons we excluded maternal transports is that they come from farther away and are possibly exposed to different weather conditions. Also, they're exposed to different sorts of medical practice. However,
O
most of our deliveries are from the city of Worcester, and by excluding maternal transports we hoped to minimize the effect of varying barometric pressures, although, as you know, New England is rather hilly and even within the city of Worcester there are some variations in pressure because of altitudes.
N THE MOVE weeks ahead
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Key words: Barometric pressure, atmospheric pressure, weather, labor, onset of labor, pregnancy
During the past two decades there have been a n u m b e r of articles that have e x a m i n e d the effect of changes in weather on h u m a n parturition. The majority of these studies e x a m i n e d the relationship between declining barometric pressure and premature spontaneous rupture of the fetal membranes. ~-4 All the studies identified a statistically significant association between these two events. In the English literature only one recent study e x a m i n e d the relationship between the onset of uterine contractions and barometric pressure changes. Driscoll and Merker ' r e p o r t e d an association between the spontaneous onset of labor and those days when the barometric pressure was low. This association was statistically significant but was based on an assmnption that did not include daily fluctuations in the size of the population of pregFrom the Department of Obstetrics and Gynecology, University of Massachusetts Medical ,School," and the Cable News Network. b Presented by invitation at the Fourteenth Annual Meeting of the Anwrican Cqnecological and Obstetrical Sodety, Napa, California, September 7-9, 1995. Reprint requests: Kenneth L. NolleT; MD, Department of Obstetrics and (~necolog3, University of Massachusetts Medical Cent~ 119 Belmont St., Wcrrchest~ MA 01605. Copyright 9 1996 by Mosby-Year Book, Inc. 0002-9378/96 $5.00+ 0 6/6/71595 1192
nant womenf' O t h e r studies have investigated whether an association between changes in the weather and eclampsia or phases of the m o o n and either delivery or spontaneous onset of labor could be identified. 6' 7 N o n e of these three studies could find such associations. All of the studies, whether positive or negative, suffered from relatively small sample sizes and the use of statistical techniques that did not adequately address the many problems encountered with such research. In spite of the absence of meaningful scientific information linking the onset of uterine contractions to changes in atmospheric pressure, it has b e c o m e increasingly c o m m o n for labor and delivery physicians and nursing staffs to believe that weather changes can cause an increase in the n u m b e r of w o m e n admitted for labor. The purpose of this investigation was to d e t e r m i n e whether an association exists between the spontaneous onset of labor in term pregnancy and changes in atmospheric pressure. If such an association were identified it could be of more than anecdotal significance. In this era of strict cost control in medicine in the United States, labor and delivery nursing staffs and other support personnel have often been downsized dramatically. The ability to predict, with some reliability, the n e e d for additional nursing staff
Volume 174, Number 4 AmJ Obstet Gynecol
some hours in the future has the potential to be of great clinical and financial benefit. Material and m e t h o d s
All deliveries occurring on the obstetric service of the Medical Center of Central Massachusetts-Memorial Hospital from Oct. 1, 1993, through Oct. 31, 1994, represent the case material for this study. Although the duration of interest was the first 12 months of the period, deliveries during the thirteenth month were abstracted to provide information for the probability calculation that was necessary for the statistical analyses (see below). A total of 3813 deliveries occurred during the 12-month period, Oct. 1, 1993, to Sept. 30, 1994. Because a copy of the maternal labor and delivery record is inserted into each newborn's chart, it was possible to obtain information from this source in those few cases where the maternal record was not available. Maternal age, last menstrual period, estimated date of confinement (EDC), date of birth, gravidity, parity, date and time of onset of contractions, date and time of spontaneous rupture of membranes, date and time of delivery, sex of infant, type of delivery, and all obstetric interventions were abstracted from the original records. In most cases the information was abstracted from the nurses' labor and delivery notes. When information was missing from this source, the senior resident admission note, the j u n i o r resident history and physical examination, the attending physician's notes, and other miscellaneous information found in the chart were abstracted, in that order. Maternal transports were culled from the data set because they represented women from practices other than those at Medical Center of Central Massachusetts-Memorial Hospital. In addition, the amount of available data concerning the EDC was variable, and most were not eligible because of preterm onset of labor or use of tocolyric agents. In addition, although most maternal transports still resided within a 50-mile radius of Medical Center of Central Massachusetts-Memorial Hospital, in general they lived farther away from the obstetric service and thus might be exposed to different climatic conditions than those women that comprise the "natural" delivery population. Changes in atmospheric conditions are recorded hourly by the National Weather Service located at the Worcester Municipal Airport. Copies of the daily logs of these readings were obtained for the study period. In addition, these data were obtained from the National Oceanographic and Atmospheric Agency, National Climatic Data Center, Asheville, N.C., in computer-readable format. The daily logs were used to supply data points missing from the computer tape. Barometric pressure recordings were taken and recorded hourly and represent the sole atmospheric measure used in this study.
Noller, Resseguie, and Voss 1193
Any attempt to detect an effect of atmospheric pressure on the initiation of labor requires some method of specifying the expected n u m b e r of initiations of labor in the absence of the weather influence. This study was restricted to women who had spontaneous onset of labor after completion of 37 weeks of gestation and before completion of 42 weeks of gestation. Therefore all women who had onset of labor between 37 and 42 weeks' gestation between Oct. 1, 1993, through Sept. 30, 1994, compose the cohort of interest. However, for calculation of the expected n u m b e r of onsets of labor it was necessary to include those women who began labor during the first 13 days after the end of the study year. For each of the 365 days of the study period, the gestational age was determined for each of the study women who potentially could have began term labor on that date. The expected n u m b e r of initiations can be determined by the formula
~12=_21P,N, where tis the gestational age relative to the EDC (t_21 = 37 weeks, t+l.~= 41 weeks 6 days), Ptis the calculated probability of initiation of labor on gestational day t, and N t is the number of women in the pool with gestational age t on that date. Least-squares regression was used to develop an equation that expresses the probability of initiation of labor for any women in our study cohort for any given gestational age. The probability model was then used to calculate the expected n u m b e r of onsets of labor that would be expected to occur on each day during the study year. The mean barometric pressure for each calendar day was calculated, and the onsets of labor occurring during the lowest tercile of pressure were compared with those occurring in the other two terciles. Ranking of mean barometric pressure by terciles was used to allow direct comparison of our findings to those of Driscoll and Merker. 5 The t test was performed to compare the square root of the ratio of observed to expected daily events in the lowest 122 days to the remaining 243 days. The square root transformation of the data was necessary to achieve a normal distribution, which is necessary for the t test. The Wilcoxon rank-sums test was also applied to these data. The atmospheric pressure reading for each of the 8760 hours of the year was categorized by the changes in atmospheric pressure over the preceding 3 hours. The prior 3 hours were stratified as showing a continuous change in an upward direction, a continuous change in a downward direction, or some other combination. The ~2 statistic was used to test the hypothesis that the onset of labor was not associated with 3 consecutive hours of falling barometric pressure. A similar but separate test was used for the hypothesis that labor was not associated with consecutively rising barometric pressure.
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Table I. Deliveries at Medical C e n t e r of Central M a s s a c h u s e t t s - M e m o r i a l Hospital, Oct. 1, 1993, to Oct. 31, 1994
Remaining sample Total deliveries abstracted Failure to meet eligibility criteria* Labor onset not in 12month study period Hour of labor onset not known
1495
4142 2647
212
2435
31
2404
*Cesarean section or induction before labor, use of tocolysis, maternal transport.
All statistical analyses were p e r f o r m e d with SAS software package (SAS Institute, Cary, N.C.) for p e r s o n a l computers.
Results Table I shows the total n u m b e r o f deliveries o c c u r r i n g d u r i n g the study p e r i o d a n d the n u m b e r s a n d reasons for exclusion. T h e h o u r of o n s e t o f labor was k n o w n for 2404 deliveries that m e t the inclusion criteria for t e r m spontan e o u s o n s e t o f labor for the 3-hour pressure c h a n g e analysis. A total of 2435 w o m e n were available for the analysis of daily b a r o m e t r i c pressure because 31 additional w o m e n h a d i n f o r m a t i o n c o n c e r n i n g the day of o n s e t b u t lacked h o u r i n f o r m a t i o n . T h e m e a n n u m b e r of l a b o r onsets p e r day was 6.67, with a r a n g e of 1 to 16. Least-squares regression was used to develop a p r o b ability e q u a t i o n that c o u l d b e used to d e t e r m i n e a n exp e c t e d n u m b e r of l a b o r o c c u r r e n c e s d u r i n g e a c h of the 365 days of the o b s e r v a t i o n period. T h e e q u a t i o n that best d e s c r i b e d the probability for the 365-day o b s e r v a t i o n p e r i o d was
1 n(p) = -2.208542 + 0.128741 [CA] - 0.000381 [GA] 2 w h e r e [GA] is the gestational age in days, with gestational age 0 days at EDC. With this f o r m u l a t h e r a n g e of exp e c t e d initiations o f l a b o r p e r day was 3.94 to 9.57. Alt h o u g h t h e r e a p p e a r s to b e large variability in the o b served n u m b e r s of l a b o r onsets c o m p a r e d with the n u m b e r e x p e c t e d by the probability model, this difference is n o t statistically significant (p = 0.84). T h e fit b e t w e e n the probability m o d e l a n d the observed probability o f a s p o n t a n e o u s o n s e t of labor is shown graphically in Fig. 1. T h e only m a j o r deviation b e t w e e n the m o d e l a n d the o b s e r v e d occurs at 41 weeks 6 days of gestation, a n e x p e c t e d f i n d i n g because, by definition, all w o m e n in the study c o h o r t m u s t have h a d o n s e t of l a b o r by the last day, whereas in the probability' m o d e l it was possible for p r e g n a n c y to c o n t i n u e b e y o n d 42 weeks. Table I1 shows the results of the analysis of the lowest tercile of average a t m o s p h e r i c pressure c o m p a r e d with the o t h e r two terciles. O n the 122 days with the lowest average b a r o m e t r i c pressure, the probability of a sponta-
n e o u s o n s e t of labor was n o t statistically significantly dit: f e r e n t from days with h i g h e r pressure. This conclusion was similar w h e t h e r the t test or the Wilcoxon two-sample test was applied. However, the sample size (122 : 243 days) was n o t sufficiently large to detect a difference less t h a n ~/Observed Expected
0.057
with a power of 80%, with ~ = 0.05. We also e x a m i n e d the m o s t e x t r e m e low-pressure days by p l o t t i n g the ratio of o b s e r v e d to e x p e c t e d to the m e a n b a r o m e t r i c pressure; we f o u n d n o excess onsets of labor. Similarly, w h e n we fitted quadratic or cubic equations to the data in a n a t t e m p t to d e t e c t a n o n l i n e a r trend, n o n e was identified. Table III shows the o c c u r r e n c e of s p o n t a n e o u s onset of l a b o r o n the basis of 3-hour periods of pressure change. T h e r e was a statistically significant deficit in the occurr e n c e of s p o n t a n e o u s onsets of labor in the h o u r after c o n t i n u o u s 3-hour pressure decreases. However, n o association b e t w e e n the s p o n t a n e o u s o n s e t of labor at t e r m a n d increasing 3-hour b a r o m e t r i c pressure was observed. Sample size calculations were p e r f o r m e d to d e t e r m i n e w h e t h e r this study h a d sufficient power to detect such an association if it existed. Because of the large n u m b e r of observations in the study, t h e r e was sufficient power to d e t e c t a difference b e t w e e n o b s e r v e d a n d e x p e c t e d p r o b abilities for these variables at the 0c = 0.05 level with 80% certainty, with a difference between o b s e r v e d a n d exp e c t e d p e r c e n t a g e s as small as 0.5.
Comment O u r data failed to d e m o n s t r a t e an increase in the occ u r r e n c e of s p o n t a n e o u s o n s e t o f labor with falling barometric pressure. This is in c o n t r a s t to the c o m m o n l y held b e l i e f t h a t a strong a t m o s p h e r i c pressure d r o p causes labor to begin. O u r findings also are in c o n t r a s t to those of Driscoll a n d Merker, 5 w h o identified a statistically significant excess in t h e o n s e t of labor d u r i n g those days in which the m e a n b a r o m e t r i c pressure was low (lowest tercile). However, t h e r e a p p e a r to b e limitations to t h e i r study. First, they did n o t a c c o u n t for the daily variation in the size of t h e i r p r e g n a n t p o p u l a t i o n . Because the n u m b e r of w o m e n who are delivered o n a given day affects the n u m b e r o f w o m e n eligible for delivery o n the n e x t ( a n d every s u b s e q u e n t ) day, this seems to b e a deficiency of t h e i r analysis. We p r e f e r r e d to c o m p a r e actual labor onsets to onsets e x p e c t e d f r o m e n u m e r a t i o n of the specific p o p u l a t i o n of p r e g n a n t w o m e n p r e s e n t o n each day. Seco n d , it is n o t possible to d e t e r m i n e the n u m b e r of actual observations o n which the study of Driscoll a n d M e r k e r was made. T h a t is, it is n o t possible to d e t e r m i n e how m a n y w o m e n went into l a b o r d u r i n g the 2 4 - m o n t h period o f t h e i r study. Third, because the n u m b e r of w o m e n delivered o n t h e i r obstetric service was increasing d u r i n g
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42
GESTATIONAL AGE IN WEEKS
Fig. 1. In (probability) of labor onset on a given day versus gestational age in weeks, where plus sign is observed probability of onset and solid line is derived probability function (see text).
T a b l e II. S p o n t a n e o u s o n s e t o f labor: Lowest m e a n a t m o s p h e r i c p r e s s u r e tercile (days) c o m p a r e d with o t h e r terciles
Days (No.)
Mean SQRTRAT
Lowest tercile
122
0.965985
Other
243
0.990066
t test* p = 0.26
Wilcoxon (rank sum) test? p = 0.37
SQRTRAT, Square root of ratio of observed to expected number of labor onsets. *Test of significance of difference between means, t = 1.135, 363 degrees of freedom. ]-Wilcoxon (rank-sums) test, S= 21473.0.
t h e study p e r i o d , the a u t h o r s a p p a r e n t l y " a d j u s t e d " t h e n u m b e r o f w o m e n b e i n g d e l i v e r e d u p w a r d to d e v e l o p a l i n e a r t r e n d for t h e i r p r o b a b i l i t y calculations. Because o t h e r a u t h o r s h a d d e m o n s t r a t e d b o t h a n increase in s p o n t a n e o u s r u p t u r e o f m e m b r a n e s a n d o n s e t o f l a b o r to b e associated with a t m o s p h e r i c p r e s s u r e decreases, we were s u r p r i s e d to find a significant d e c r e a s e in t h e o n s e t o f l a b o r after 3 - h o u r intervals w h e n a t m o s p h e r i c p r e s s u r e was d r o p p i n g . However, b e c a u s e o f the
T a b l e III. S p o n t a n e o u s o n s e t o f l a b o r in h o u r after 3 - h o u r interval with c o n t i n u o u s l y increasing, decreasing, o r o t h e r a t m o s p h e r i c p r e s s u r e c h a n g e s
Pressure change
No. of 3 hr intervals
No. of 3 hr intervals
Down Up Other
1809 (20.7%) 1799 (20.5%) 5152 (58.8%)
440 (18.3%)* 493 (20.5%)]1471 (61.2%)
*Continuously down pressure versus all others: Z2= 8.0872, 1 degree of freedom; p < 0.005. ]-Continuously up pressure versus all others: ;(2 = 0.0012, 1 degree of freedom; p > 0.97. relatively large n u m b e r o f o b s e r v a t i o n s (labor onsets) in o u r study, t h e c o n c e p t o f "statistical significance" is perh a p s misleading. In large studies even a m i n o r deviation f r o m e x p e c t e d can result in a small p value. N o n e t h e l e s s , we o b s e r v e d a deficiency in the o n s e t o f l a b o r >10% of the expected. T h e o n s e t o f l a b o r is k n o w n to vary in a d i u r n a l m a n ner.8. 9 In these studies t h e p e a k o n s e t o f l a b o r o c c u r r e d b e t w e e n m i d n i g h t a n d 2 AM. W h e n we e x a m i n e d o u r data for t h e time o c c u r r e n c e o f l a b o r onset, we also o b s e r v e d t h a t m o s t of t h e labors b e g a n b e t w e e n m i d n i g h t a n d 4 AM.
1196
Noller, Resseguie, and Voss
April 1996 ,~nJ Obstet Gynecol
52
52
50
x of
days w i t h e n d i n g at
48 46
3 hour d r o p p i n g hour of day
z o n s e t s of labor N = 2404
44
during
5O
pressure
48 46
hour
44
,12
42
40
40
;8
38 36
;4-
34
;2-"
32 ~0
2
28
26 24
26
22
22
20
20
18
18
16
16
24
J
14 12 10 8
14 12 10 8
6
6
4
4
2
i
i
i
i
i
I
I
I
t
I
J
I
I
i
I
I
I
I
I
I
I
I
1
2
3
4
5
6
7
8
9
10
11
12
1,3
14
15
16
17
18
19
20
21
22
2 2,3
Fig. 2. Percent versus h o u r of day where plus sign is percentage of days with 3-hour continuously dropping pressure ending at h o u r of day a n d circle is percentage of onsets of labor during h o u r (n = 2404).
When we c o m p a r e d the fraction of labor onsets that occurred at any given h o u r with the barometric pressure change for the preceding 3 hours for the same h o u r of the day, we found an interesting association (Fig. 2). As can be seen from this figure, the peak occurrence of decreased barometric pressure roughly coincides with the nadir of labor onset. W h e t h e r this is a chance association, an association resulting from a third factor (e.g., absence of light), or whether there is a cause-and-effect relationship is impossible to say. However, a considerable n u m b e r of physiologic changes do occur on the basis of circadian rhythm, and uterine activity should not be excluded from consideration. O n e of the reasons this study used the time period Oct. 1, 1993, to Sept. 30, 1994, is the fact that it coincided with one of Worcester County's most meteorologically unstable years. The winter was plagued by several large snow storms (Nor'easters) with a total accumulation of snow to a depth of nearly 10 feet. We hypothesized that, if the onset of labor were associated with the passage of a "front," the selected time period might have identified such an association. Unfortunately, although frontal theory remains an integral part of meteorologic thought, this theory, first de-
veloped by a group of Norwegian scientists during World War I, defies explicit definition. Fronts are generally defined as the boundaries separating air masses of different densities. Such definition implies a temperature contrast, atmospheric pressure troughs, changes in wind direction, moisture discontinuity, and certain characteristic cloud and precipitation forms. T h e r e are warm fronts, cold fronts, and stationary fronts. Some fronts are associated with a rapid drop in air pressure, but there are times when a weather front passes without appreciable change in atmospheric pressure. O n the other hand, some s t o r m s - - s u c h as the classic New England " N o r ' e a s t e r " can induce an intense drop in pressure without the observation of a frontal passage as defined by a meteorologist. Thus there were no combinations of information collected from the Worcester Weather Station that could be used to write a program that would identify a weather "front." Indeed, meteorologists identify weather fronts by looking at the current and previous recordings at their weather station and they identify a front and its passage more by experience than by definition. Although this is the largest and most exhaustively analyzed study of the relationship between the spontaneous onset of term labor and atmospheric pressure, our restdts
Volume 174, Number 4 AmJ Obstet Giqleco]
do not absolutely p r e c l u d e the possibility that pressure changes also can be positively associated with the onset of labor in o t h e r geographic areas. O u r data are reliable only for o u r specific hospital, for a specific year, with the practice patterns in place at the time of the study. REFERENCES
1. Polansky GH, Varner MW, O'Gorman T. Premature rupture of the membranes and barometric pressure changes. J Reprod Med 1985;30:189-91. 2. Milingos S, Messinis I, Diakomanolis D, Aravantinos D, Kaskarelis D. Influence of meteorological factors on premature rupture of fetal membranes. Lancet 1978;2:435. 3. Steinman G, Kleiner G. Spontaneous premature rupture of membranes: correlation with ambient atmospheric pressure. N Y State J Med 1978;78:900-1. 4. Boland N, Byers R, Kitay D. Meteorologic phenomena and labor initiation. Atlanta (GA): Department of Gynecology and Obstetrics, Emory University School of Medicine, 1980: 168. Bulletin No. 2. 5. Driscoll D, Merker D. A search for associations between weather and the onset of human parturition. IntJ Biometeorol 1984;28:211-24. 6. Alderman BW, Boyko EJ, Loy GL, Jones RH, Keane EM, DalingJR. Weather and o c c u r r e n c e of edampsia. IntJ Epidemiol 1988;17:582-8. 7. Witter E The influence of the moon on deliveries. A_,~J OBs'rEz GWECOL1983;124:637-9. 8. Cooperstock M, England J, Wolfe R. Circadian incidence of labor onset hour in preterm birth and chorioamnionitis. Obstet Gynecol I987;70:852-5. 9. Fraser W, McLean F, Usher R. Diurnal variation in admission to hospital of women in labour. CanJ Surg 1989;32:33-5.
Discussion
DR. ROBERT C. CEFALO, Chapel Hill, North Carolina. Centuries ago the a p p o i n t e d time of labor was declared to be a law of God; many hypotheses have since a t t e m p t e d e n l i g h t e n m e n t , only to be displaced in time by others. We now know that at a species-specific time in gestation the growing fetus decreases its d e p e n d e n c e on the nutritional, respiratory, excretory, and e n d o c r i n e functions of the placenta, and parturition ensues. In most m a m m a l i a n species the multifactorial contributions of the fetus-placenta and m o t h e r to the onset of labor appear to be interrelated. Transient increases in fetal adrenal cortical secretion, changes in placental horm o n e p r o d u c t i o n relating to the progesterone and estrogen ratio, release o f phospholipase and arachidonic acid by the placental m e m b r a n e s , increased synthesis and release of relaxin and prostaglandins by the uterine epithelium, and increased myometrial sensitivity to ecbolic agents such as oxytocin and prostaglandin F2,~ have all b e e n implicated in the c o n u n d r u m of the onset of labor. T h e r e appears to be a disproportionate n u m b e r of events in the fetus that occur just before labor with only m i n o r maternal influences. 1 T h e m o t h e r ' s control is similar to her power over onset of puberty, adolescent impulses, or selection of a mate. But there are maternal factors: mares foal in darkness, rats deliver in daylight, and rabbits will not deliver u n d e r direct observation. T h e alpaca, an animal that resides high in the Andes, delivers between 5 ~\~ and 2 PM to take advantage of the sunshine; there are no births on very cold days. l' 2 In humans, controversy surrounds the question of
Noller, Resseguie, and Voss
1197
whether spontaneous labor, p r e m a t u r e r u p t u r e of the membranes, or birth has a 24-hour rhythmicity and, if so, what role e x o g e n o u s factors may contribute. 3" ~ For many years there has b e e n an attempt to link m e t e o r o l o g i c factors such as weather, barometric pressure change, seasonal changes, tides, and m o o n phases to the onset of p r e m a t u r e r u p t u r e of membranes, p r e t e r m labor, labor, and h i g h e r prevalence of preeclampsia-eclampsia. Hippocrates wrote, " T h e changes of seasons mostly eng e n d e r diseases, and in the same season heat or cold could create great changes. ''~ In 1756 Smellie observed a clustering of cases of eclampsia and attributed them to changes in the weather. Many o t h e r writers have shared his opinion. In 1984 Chesley 6 f o u n d that the incidence of preeclampsia reached a peak in March and April; the meteorologists cited unsettled weather as the cause, whereas others believed that an increased n u m b e r of brides the previous J u n e had provided a surge of primigravidas. Anecdotal observations have b e e n m a d e linking the onset of labor and " b a d weather" conditions. Because the a p p r o a c h o f " b a d weather" is often a c c o m p a n i e d by a drop in atmospheric pressure, several authors have studied but not f o u n d a consistent relationship to changes in barometric pressure and the onset of labor. However, some authors cited by Noller et al. 5' 7 did find that a lower m e a n barometric pressure may be related to a h i g h e r prevalence of p r e m a t u r e r u p t u r e of the membranes. In addition to barometric pressure, o t h e r e n v i r o n m e n tal considerations may m o d u l a t e the process of parturition, such as variability in geographic latitude, climate, temperature, length of light-dark cycles, light intensity, and work schedules. T h e presence of the full m o o n has been ascribed powers to foster mental illness, crime, or an increase in the n u m b e r of births; fortunately, these ideas have b e e n misbegotten and unproved. However, there is a synchronicity of lunar phases with the rise and fall of the ocean tides along with the m o o n ' s ability to influence the weather through the creation of daily changes in the air pressure; this, in part, explains the coincidence of lunar phases with cloudiness, rainfall, and hurricane formation. T h e full m o o n may n o t affect your sanity, but it apparently unbalances the weather, in turn, the weather may be associated with barometric pressure changes that may be associated with the onset of l a b o r - but b e h o l d the study by Noller et al. T h r o u g h a sophisticated statistical analysis of the data, the authors have shaken my belief that the onset of labor is associated with falling barometric pressures and that the full m o o n brings on m o r e deliveries than o t h e r lunar phases. However, they c a n n o t take away the fact that blackouts, blizzards, typhoons, hurricanes, and o t h e r weather extremes do lead to an increased n u m b e r of deliveries approximately 40 weeks later! I must now ascribe my labor deck experiences not to fact but to bad luck. Howevel, as sophisticated the study by Noller et al. was, I have several questions. 1. O u r definition of clinical labor is the time of regutar contractions leading to the birth of a child. How do the authors account for the changes in the cervix and lower uterine s e g m e n t that have b e e n associated with regional uterine activity? How would the authors relate
1198 Noller,Resseguie, and Voss
.\p, il 1996 \ m J ()bSlet (;vnecol
these c h a n g e s to o n s e t of labor a n d b a r o m e t r i c pressure readings? 2. T h e a u t h o r s readily a d m i t that o t h e r e n v i r o n m e n t a l c o n s i d e r a t i o n s may m o d u l a t e the process of parturition, such as variability in g e o g r a p h i c latitude, prior p h o t o p e riod e x p e r i e n c e , light intensity, a n d work schedules. How d o the a u t h o r s a c c o u n t for these potential variables? 3. T h e a u t h o r s fail to m e n t i o n how the EDC was determ i n e d . How would a n inaccuracy affect their resuhs? 4. Is the fluctuation of b a r o m e t r i c pressure within a day (e.g., day vs night) larger t h a n fluctuation a m o n g days? Averaging b a r o m e t r i c pressure for each day may to some e x t e n t dilute the effects of low pressure. If this is true, t h e n prior 3-hour c h a n g e s in pressure m i g h t be a m o r e sensitive m e a s u r e m e n t t h a n a m e a n pressure for the day.
REFERENCES
1. I,iggins GC. Initiation of parturition. Br Med Bull 1979;35: 145-50. 2. Nathanielsz PW. The tketus and birth. Amsterdam: Elsevier, 1977:151-2. (Ciba Foundation no. 47.) 3. Ixmgo LD, Yellon SM. Biological timekeeping during pregnancy and the circadian rhythms in parturition. In: Kunzel W, Jensen A, eds. The endocrine control of the fetus. Berlin: Springer-Verlag, 1988:17.9,-.92. 4. Honnebrew MBOM, Nathanielsz PW. Primate parturition and the role of maternal circadian system. Eur J Obstet Gynaecol Reprod Biol 1994;55:193-203. 5. Bider"O, Sivan E, Seidman DS, et al. Meteorological factors in hypertensive disorders, vaginal bleeding and premature rupture of the membranes during pregnancy. Gynecol Obstet Invest 1991 ;32:88-90. 6. Chesley LC. History of the epidemiology of preeclampsia and eclampsia. Clin Obstet Gynecol 1984;27:801-20. 7. Steinman G, Kleiner G. Spontaneous rupture of" membranes--cnrrelation with ambient atmospheric pressure. N Y State.] Med 1978;5:900-1.
De,. MICHAEL C,. Ross, Los Angeles, California. H e a t stress has b e e n associated with catecholatnine, vasopressin, a n d oxytocin release, in a d d i t i o n to plasma volu m e contraction. Because m e t e o r o l o g i c data were available, did you control for the m e a n daily t e m p e r a t u r e ? DR. THOMAS H. KIaSCn~AUM, Bronx, New York. T h e r e is an implicit a s s u m p t i o n in the quadratic probabilio, function that you d e f i n e d t h a t the o n s e t of l a b o r follows b a r o m e t r i c c h a n g e s within 24 hours, a n d the t r e n d analysis makes the a s s u m p t i o n that t h e r e ' s little delay in the o n s e t of labor. You m i g h t c o n s i d e r looking at the possibility, that l o n g e r intervals or s h o r t e r intervals between b a r o m e t r i c events a n d o n s e t of labor may show something. DR. J o n s T. R ~ I ~ , Boston, Massachusetts. I can r e m e m b e r f i n d i n g an old article from the mid-1960s in which C a v a n a u g h suggested that the i n c i d e n c e o f p r e e c l a m p s i a a n d eclampsia in south Florida m i g h t c h a n g e d u r i n g h u r r i c a n e season. O n e of the issues I raised with a m e t e o rologist friend of m i n e at the time was how m u c h variabilitT is t h e r e in b a r o m e t r i c pressure from o n e location to a n o t h e r . W h a t is the proximity of the l a b o r a n d delivery u n i t a n d even the location of the patients' h o m e s to the Worcester airport, a n d how do you really know that your o b s e r v a t i o n is a reflection of the b a r o m e t r i c pressure t h a t
you were b e i n g given f i o m the m o r e r e m o t e location in Worcester? De,. MmnaEl. L. SOCOL, Chicago, Illinois. Because a significant p e r c e n t a g e of patieuts at t e r m have r u p t u r e d m e m b r a n e s b u t are n o t in labor, I'm interested to know how yon dealt with those patients in your analysis. 1 know a n u m b e r of patients were excluded, b n t it was unclear how many h a d s p o n t a n e o u s r u p t u r e of m e m b r a n e s n n t in labor. DR. NOLLER (Closing). Dr. Cefhlo asked how we acc o u n t e d for p r e l a b o r c h a n g e s of the cervix in o u r analysis. T h e r e ' s really, n o g o o d answer to that. We certainly c a n n o t postulate that w e a t h e r c h a n g e s a c c o u n t for the p r e t e r m c h a n g e s in cervical dilatation a n d e f f a c e m e n t that so m a n y w o m e n u n d e r g o . We suggest only that the trigger for the onset of labor, whatever t h a t is, m i g h t be i n f l u e n c e d in some way by c h a n g e s in the weather. Yon also asked a b o u t the o t h e r things, such as work, light, a n d latitude, that can affect the onset of labor. In o u r full article we discuss h o u r s of the day; t h e r e deftnitely is cyclicity. N i n e t e e n different studies have shown that the most f r e q u e n t time of the onset of labor is belween m i d n i g h t a n d 4 .~M, with a real p r e f e r e n c e for m i d n i g h t to 2 xx~. We also identified t h a t o c c u r r e n c e in o u r data. T h e r e are two answers to the question a b o u t o t h e r variables. O n e is pragmatic a n d o n e is statistical. Pragmatically, we c a n ' t a c c o u n t t o t all the variables. We can only a c c o u n t for those we t h o u g h t about, a n d I would suggest that n o one, in any study, can a c c o u n t for all the variables. T h e statisticians, of course, have a second answer to this question. They include all the o t h e r variables that nobody's t h o u g h t of in the " e r r o r variance." T h e waywe calculated the EDC was as I m e n t i o n e d . Is it accurate? Certainly it is not. In fact, I t h i n k it can be a r g u e d t h a t t h e r e is n e v e r a n accurate EDC. W h a t we h o p e is that, because we used a consistent m e t h o d , the errors in the calculation of the EDC were r a n d o m r a t h e r than systematic a n d t h e r e f o r e t h e r e s h o u l d n o t be any systematic bias i n t r o d u c e d into the results. In answer to the n e x t question, b a r o m e t r i c pressure did vary within days a n d f i o m day to day d u r i n g the year, T h e longest variation within any single day was similar to the m a x i m u m c h a n g e t h r o u g h o u t the year. T h e c o m m e n t t h a t using the m e a n daily b a r o m e t r i c pressure is an insensitive m e a s u r e is certainly true. W h e n we used the m e a n daily b a r o m e t r i c pressure, we f o u n d a small decrease in the o b s e r v e d versus the e x p e c t e d o n the low pressure days, b u t it wasn't significant. It was only w h e n we looked at 3-hour intervals t h a t we t o u n d a significant difference. 1 would agree that m e a n daily barometric pressure is insensitive. H e a t stress a n d t e m p e r a t m e c h a n g e s could be implicated. We have n o t looked into this. It's o n e of the bits of i n f o r m a t i o n we have f r o m the w e a t h e r service, a n d it is o n e of the things we h o p e to look at in the future. We did look at l o n g e r a n d s h o r t e r intervals, such as those in the U n i v e r s i ~ of Iowa study. They l o o k e d at 3, 6, 9, a n d 12 hours. We also l o o k e d at those, b n t because the 3-hour intervals were the most significant in t h e i r study it
Noller, Resseguie, and Voss 1199
Volume 174, Number 4 AmJ Obstet Gynecol
was the one that meant the most to us. We postulated that if weather was going to have an effect, it would be shortly before the onset of labor. One of the reasons we excluded maternal transports is that they come from farther away and are possibly exposed to different weather conditions. Also, they're exposed to different sorts of medical practice. However,
O
most of our deliveries are from the city of Worcester, and by excluding maternal transports we hoped to minimize the effect of varying barometric pressures, although, as you know, New England is rather hilly and even within the city of Worcester there are some variations in pressure because of altitudes.
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