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Are Hong Kong babies getting bigger?
International
Journal
of Gynecology
& Obstetrics
57 (1997)
267-271
Article
Are Hong Kong babies getting bigger? G.M. Brieger, M.S. Rogers*, A.W. Rushton, M. Mongelli Depaments
of Obsteks
Received
and Gynaecology
23 October
and Diagnostic Radiology, The Chinese New Territories, Hong Kong 1996; revised
24 February
1997; accepted
Uniuersity
of Hong
5 March
1997
Kong, Shatin,
Abstract Objective: To establish recent birth weight trends in Hong Kong. Method: A total of 10512 confinements for the years 1985-86, and 7857 for the years 1995-96 were analyzed. Result: There was a significant increase in maternal height, weight at booking, and maternal age, whereas the body-mass index was reduced slightly (P < 0.0001). Parity increased significantly, with the percentage of parous women rising from 44.1% to 55.6% (P < 0.0001). The percentage of female infants decreased from 49.5% to 47.9%. Despite these changes there was no significant difference in mean birth weights between the two groups. When birth weight was controlled for sex, parity, maternal height and weight there was a trivial increase of 15 g over time (P = 0.01). Conclusion: Birth weight has reached a plateau in Hong Kong despite a continuing increase in the regions’ socioeconomic status, and evidence of improved nutritional well-being. 0 1997 International Federation of Gynecology and Obstetrics Keywords:
Birth weight; Hong Kong
1. Introduction Birth
weight
related remains
a significant
epidemiolog-
ical index of reproductive performance, and is
*Corresponding UO20-7292/97/$17.00 PIZ SOO20-7292(97)
author. 0 1997 International 00068-4
Federation
of Gynecology
to a large
number
of variables
[l]. It has
rightly been suggested that each obstetric center should consider establishing or revising its own standards so that up-to-date birth weight reference charts may be maintained 121. This was advocated particularly by clinicians in Hong Kong because of the rapid changes in the and Obstetrics
268
GM
Brieger et al. /International
Journal
socioeconomic and nutritional state of the population [l]. Birth weight in the Territory has been of interest because of the reported change in distribution [l] and has been the subject of several studies [l-4]. The early reports suggest that Hong Kong Chinese babies are smaller than Caucasian babies at term [3,4], whilst the later publications conclude that the median birth weights resemble those in other Caucasian and Asian reports [2]. What remains unclear, however, is whether, once a favorable socioeconomic environment is established, mean birth weights will continue to increase or level off in the long term [S]. Observational studies in adults indicate that growth in response to environmental factors will reach a peak and then plateau [5]. It is pertinent therefore to determine whether birth weight in Hong Kong Chinese has continued to increase over the last decade. 2. Materials
and methods
Obstetric records of all confinements in the Prince of Wales Hospital, Hong Kong, were entered into a database for two periods, January 1985 to December 1986, and January 1995 to June 1996, using items previously described [l]. For this analysis, we excluded congenital malformations, stillbirths, multiple pregnancies, birth weights less than 500 g, and those who were not Hong Kong Chinese. Birth weight was measured to the nearest 50 g, babies being weighed at birth on scales (Tanita). Gestational age was derived from the best clinical estimate, based on the last menstrual period, date of first positive pregnancy test, and ultrasound measurements, where these were considered suspect, and expressed as completed weeks of pregnancy. Where applicable, gestational age was corrected by neonatal assessment based on the Dubowitz scoring system [6] for healthy infants, or the Farr scoring system [7] where infants required intensive care. Neither of these scoring systems are birth weight-dependent [ 11. Maternal age was recorded as the number of completed years at booking. Maternal height was measured in cm and weight at booking in kg. Parity was
of Gynecology
& Obstetks
57 (1997) 267-271
defined as the number of previous pregnancies with a gestation of 24 or more weeks. Immigrant status was attributed to women who had lived in Hong Kong for 10 years or less, and these were excluded. Maternal smoking was not analyzed as less than 1% of women delivered in the unit admitted to smoking [l]. 2.1. Statistical methods The data were analyzed in 2 groups representing the extremes of a decade. The first group consisted of all babies born between 1985-86, while the second included deliveries in 1995-96. For each group, the frequency distributions of birth weight, maternal age, gestational age at delivery, parity and fetal sex were determined (Table 1). Differences in means between groups for birth weight, maternal age, maternal weight at booking, maternal height and gestational age at delivery were calculated using Student’s t-test. Birth weight was further subdivided into three groups, I 1500 g, I 2500 g, and 2 4000 g. Differences in proportions between groups were assessed using the chi-square test. In order to adjust birth weight for maternal characteristics, multiple regression analysis was employed (‘enter all’ method) on the pooled data from the two groups. Independent variables included in the analysis were maternal height, maternal weight at booking, sex of the baby, parity and time group (i.e. group 1 or group 2). Gestational age was not included, for reasons given in Section 4. Categorical variables were transformed Table 1 Changes in gestational age
Mean (SD.) Number (%) post-term: 2 42 weeks Number (%) pre-term: < 37 weeks *P **P
Group 1 (1985-1986)
Group 2 (1995-1996)
39.4 (1.9) 992 (9.4)
38.9 (2.0)* 386 (4.9r**
264 (5.8)
240 (6.9r*
< 0.001 by Student’s t-test. < 0.001 by Chi-square test.
GM
Brieger et al. /International
Journal
into dummy variables prior to entry as follows. Multiparas were coded as 1, nulliparae 0. Male sex was entered as 1, female 0. Group 2 is coded as 1, group 1 is the reference (0). Cases with missing data were excluded from the analysis. To determine the factors related to the incidence of low birth weight and very low birth weight, logistic regression analysis was carried out. Independent variables included were maternal height, maternal weight at booking, gestational age, sex of the baby, parity and time group (i.e. group 1 or group 2). All statistical tests were performed with SPSS for Windows (version 7.0, SPSS Inc., CA). 3. Results
A total of 10512 confinements were reviewed for the years 1985-86 (group 11, and 7857 for the years 1995-96 (group 2). Data was missing in 17 cases, and there were no out-of-range values. Mean gestational age at delivery showed a significant decrease over the 10 years. The percentage of post-term pregnancies decreased from 9.5% to 4.8%, while preterm deliveries increased slightly from 5.8% to 6.9%. There was a significant increase in maternal Table 2 Maternal
and pregnancy
characteristics
Group 1 (1985-1986) Height (cm) Mean (SD.) Booking weight Mean (S.D.) Age (years) Mean (S.D.) BMI (kg/m*) mean (SD.) Parity (%I: Para 0 Para 1 Para 2 + Sex ratio (% females) “Student’s bChi-square
t-test. test.
Group 2 (1995-1996)
Significance: P-value
155.8 (5.3)
157.5 (5.5)
< O.OOOld
56.6 (8.7)
57.2 (8.9)
< 0.0001”
27.6 (4.5)
29.7 (5.0)
< 0.000 1”
23.3 (3.4)
23.1 (3.4)
< O.OOO1d
55.9 26.1 18.0
45.5 37.9 16.6
< O.OOO1b
49.5
47.9
0.036
of Gynecologv
& Obstettics
57 (1997) 267-271
269
height, weight at booking, and maternal age, whereas the body-mass index was reduced slightly (Table 2). Important changes in the distribution of parity and fetal sex were noted between the two groups. Parity increased significantly, with the percentage of parous women rising from 44.1% to 55.6%. The sex ratio changed significantly, so that the percentage of female infants decreased from 49.5% to 47.9%. There was no significant difference in mean birth weights between the two groups. However, when birth weight was controlled for sex, parity, maternal height and weight in the multiple regression there was a small but significant increase over time (Table 3), by 15 g. A significant increase in the number of babies weighing 5 1500 g was noted, whilst there was no change in the groups < 2500 g and in those L 4000 g. Logistic regression analysis on the factors related to very low birth weight and low birth weight showed that gestational age was the only significant variable; group membership was not significant. 4. Discussion
Birth weight is recognised as a global indicator of community health [8], and it has been shown to increase over time in developing countries [9]. This is speculatively attributed to improved environmental conditions [5,9]. Average birth weight in Hong Kong Chinese was reported in 1964 as Table 3 Changes in birth
weight Group 1 (1985-1986)
(kg)
Birth weight (g): Mean (S.D.) 3215 (465) Birth weight (g): Median (skew) 3200 ( - 0.38) Number (%) < 1500 g 51(0.49) Number (%) < 2500 g 630 (6.0) Number (%) > 4000 g 489 (4.7) “Chi-square test. hStudent’s t-test. ‘Kolgomorov-Smirnov distributions.
Group 2 (199551996)
3211 (499) 3200 75 517 381
test for difference
( - 0.81) (0.95) (6.6) (4.8)
P-value
0.57” < 0.001’ < O.OOO1b 0.109b 0.5sh
in the skew of the
270
G.M.
Brieger et al. /International
Journal
3110 g. Later, data comparing similar Hong Kong Chinese groups over a 12-year interval to 1984 found birth weight to have increased significantly at term from a mean birth weight at 40 weeks of 3130 g to 3237 g [2-41. The increase was confirmed in other series, and attributed in part to the rapid changes in socioeconomic and nutritional state [l]. Our findings suggest that over the last 10 years, mean birth weight has remained constant, whilst it is recognised that the socioeconomic and nutritional states have continued to change [lo]. We confirmed that both maternal height and weight, indicators of nutritional well-being [5], have continued to increase significantly over time (Table 2). This has occurred with a slight reduction in obesity, the BMI being significantly decreased. Birth weight is known to increase with gestational age, maternal weight, height, parity, male sex fetus, as well as increased maternal age [ill. Of these, gestational age has been shown to be the strongest variable. The changes in pregnancy characteristics that we have described, with the exception of gestational age, would have favored an increase in birth weight. So would a possible increase in the incidence of gestational diabetes [12] due to dietary changes. Multiple regression analysis allowed us to assess changes in birth weight while adjusting for sex and maternal characteristics; the results suggested an increase of only 15 g over the period studied (Table 4). This may be due to changes in cigarette consumption, as the proportion of women smokers has decreased over the last decade [13]. It therefore raises the possibility that the maximum mean birth weight has been reached for Hong Kong Chinese, despite continuing nutritional and socioeconomic growth. The significant increase in the very low birth weight group is cause for concern, as these babies have the highest rates of mortality and long-term morbidity. Whilst the numbers are not great enough to skew the distribution, and the quartile values as well as the median remain unchanged, it does suggest that the overall reproductive performance of Hong Kong women has deteriorated. The increase in assisted reproduction (although multiple pregnancy was excluded), and the in-
of Gynecology
& Obstetrics
57 (1997) 267-271
Table 4 Birth weight in relation to maternal characteristics: multiple regression analysis Variable
Coefficient
Standard error
T
P-value
Constant Age Height Weight Parity Sex (male) Group
- 3594.3 4.2 6.6 10.5 72.0 105.4 15.3
104.9 0.7 0.6 0.4 6.4 5.8 6.1
- 34.3 6.4 11.4 29.4 11.3 18.3 2.5
0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0120
Dummy variables are coded as follows: multiparas (11, nulliparae (0); male (l), female (0). Group 2 is coded as (l), group 1 is the reference (0).
Note.
crease in maternal age may account for some of the change. The increased proportion of male infants delivering in our unit over the decade was an unexpected finding. These changes are strongly suggestive of extraneous factor(s) operating during the last decade. Changes in the sex ratio attributable to increased exposure to alcohol and lead, as well as changes due to increased socioeconomic status have been described elsewhere [14,15]. Furthermore, feticide on the basis of sex-selection and the one-child policy have been described in mainland China, all of which may be factors in the population studied. Immigration from mainland China is unlikely to account for the change in mean birth weight and sex ratio, as only those women who had been resident in Hong Kong for more than 10 years were included in the analysis. The observed changes in the gestational age distribution, more preterm and less post-term pregnancies, are typically seen during a period of transition from dating by the last menstrual period to dating by ultrasound [16]. Over the time interval studied, ultrasound equipment has been used increasingly to assist in gestational age assignment. Currently, ultrasound examination at booking is reserved for those whose last menstrual period dates are unknown, uncertain or inconsistent with clinical examination. The apparent changes noted in the gestational age distribution are likely to be confounded by the method of gestational age assignment. Hence this parameter
GM
Brieger
et al. /International
Journal
was not included in the regression analysis, in contrast to the previous studies [l-3]. Analyses of similar birth weight data in Western countries have shown trends opposite to those noted in Hong Kong. When data was reviewed up to 1989 in the United States, England and Wales there had been a steady increase in the median birth weight which was attributed in part to a reduction in maternal smoking, and improved nutrition [ll]. It was also concluded that the shift towards higher birth weight should continue in Western countries. It appears that birth weight has reached a plateau in Hong Kong despite a continuing increase in the regions’ socioeconomic status, and evidence of improved nutritional well-being [17]. The analysis of this data enables an accelerated view due to the Territories rapid development, and the small but significant decrease in adjusted mean birth weight was indeed an unexpected finding. Similarly the change in sex ratio is suggestive of an unrecognized influence on this population. References ill PI
Dl [41
Rogers MS, Wong FWS, Chang AMZ. Determinants of birth-weight in the New Territories of Hong Kong. Aust NZ J Obstet Gynaecol 1987; 27: 314-319. Woo JSK, Li DFH, Ma HK. Intrauterine growth standards for Hong Kong Chinese. Aust NZ J Obstet Gynaecol 1986; 26: 54-58. Ip HMH. Intrauterine growth in Hong Kong Chinese. Biol Neonate 1978; 33: 253-263. Chun D, Kan PS, Ho HC, Wong CP. The Hong Kong Chinese female boney pelvis and its influence on labour: a radiographic and clinical study of 1005 women. J Obstet Gynaecol Br Commonw 1964; 71: 556-564.
of Gynecology
& Obstettics
57 (1997)
267-271
171
[A
Gruenwald P, Funakawa H, Mitani S, Nishimura T, Takeuchi S. Influence of environmental factors on foetal growth in man. Lancet 1967; 1: 1026-1028.
[hl
Dubowitz L. Assessment born: a practical scoring 44: 782.
[71
Farr V. Mitchell RG, Neilgan GA, et al. The definition of some external characteristics used in the assessment of gestational age in the newborn infant. Dev Med Child Neurol 1966; 8: 507~511.
Bl
Development of indicators for monitoring progress towards ‘Health for all by the year 2000’. Health for All series, No. 4. Geneva: WHO. 1981. Satpathy R, Das DB, Bhuyan BK, Pant KC, Santhanam S. Secular trend in birth weight in an industrial hospital in India. Ann Trop Paediatr 1990; 10: 21-m25.
HOI
Brieger GM, Yip SK, Chung T. Genital prolapse: a legacy of the West? Aust NZ J Obstet Gynaccol 1996: 36: 52-54.
[111
Alberman E. Are our babies Med 1991: 84: 257~260.
of gestational system. Arch
hecoming
age in the newDis Child 1969:
bigger?
J R Sot
[I21
Beischcr NA, Wein P, Sheedy MT, Stcffen 9. Identification and treatment of women with hyperglycaemia in pregnancy can significantly reduce perinatal mortality Irates. Aust NZ J Obstet Gynaecol 1996: 36: 239-247. r1.71 Public Health Report No. 1. A new approach to public health report, coronary heart disease and lung cancer. Hong Kong: Department of Health, 1944: 7-3
[151
Dickenson sex ratio?
[ISI
Ulizzi L. Relationship between earl) mortality and sex ratio: changes over the last century in Italy. Ann Hum Genet 1983: 47: 321.-327
H, Parker L. Do alcohol and lead change J Theor Biol 1994; 169: 313 315.
1161 Mongelli
the
M. Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol 1996: 174: 778-284.
1171
Hewlet Printer.
B. Hong Kong 1996: 49.
1996. H.
Myers.
Government
Journal
of Gynecology
& Obstetrics
57 (1997)
267-271
Article
Are Hong Kong babies getting bigger? G.M. Brieger, M.S. Rogers*, A.W. Rushton, M. Mongelli Depaments
of Obsteks
Received
and Gynaecology
23 October
and Diagnostic Radiology, The Chinese New Territories, Hong Kong 1996; revised
24 February
1997; accepted
Uniuersity
of Hong
5 March
1997
Kong, Shatin,
Abstract Objective: To establish recent birth weight trends in Hong Kong. Method: A total of 10512 confinements for the years 1985-86, and 7857 for the years 1995-96 were analyzed. Result: There was a significant increase in maternal height, weight at booking, and maternal age, whereas the body-mass index was reduced slightly (P < 0.0001). Parity increased significantly, with the percentage of parous women rising from 44.1% to 55.6% (P < 0.0001). The percentage of female infants decreased from 49.5% to 47.9%. Despite these changes there was no significant difference in mean birth weights between the two groups. When birth weight was controlled for sex, parity, maternal height and weight there was a trivial increase of 15 g over time (P = 0.01). Conclusion: Birth weight has reached a plateau in Hong Kong despite a continuing increase in the regions’ socioeconomic status, and evidence of improved nutritional well-being. 0 1997 International Federation of Gynecology and Obstetrics Keywords:
Birth weight; Hong Kong
1. Introduction Birth
weight
related remains
a significant
epidemiolog-
ical index of reproductive performance, and is
*Corresponding UO20-7292/97/$17.00 PIZ SOO20-7292(97)
author. 0 1997 International 00068-4
Federation
of Gynecology
to a large
number
of variables
[l]. It has
rightly been suggested that each obstetric center should consider establishing or revising its own standards so that up-to-date birth weight reference charts may be maintained 121. This was advocated particularly by clinicians in Hong Kong because of the rapid changes in the and Obstetrics
268
GM
Brieger et al. /International
Journal
socioeconomic and nutritional state of the population [l]. Birth weight in the Territory has been of interest because of the reported change in distribution [l] and has been the subject of several studies [l-4]. The early reports suggest that Hong Kong Chinese babies are smaller than Caucasian babies at term [3,4], whilst the later publications conclude that the median birth weights resemble those in other Caucasian and Asian reports [2]. What remains unclear, however, is whether, once a favorable socioeconomic environment is established, mean birth weights will continue to increase or level off in the long term [S]. Observational studies in adults indicate that growth in response to environmental factors will reach a peak and then plateau [5]. It is pertinent therefore to determine whether birth weight in Hong Kong Chinese has continued to increase over the last decade. 2. Materials
and methods
Obstetric records of all confinements in the Prince of Wales Hospital, Hong Kong, were entered into a database for two periods, January 1985 to December 1986, and January 1995 to June 1996, using items previously described [l]. For this analysis, we excluded congenital malformations, stillbirths, multiple pregnancies, birth weights less than 500 g, and those who were not Hong Kong Chinese. Birth weight was measured to the nearest 50 g, babies being weighed at birth on scales (Tanita). Gestational age was derived from the best clinical estimate, based on the last menstrual period, date of first positive pregnancy test, and ultrasound measurements, where these were considered suspect, and expressed as completed weeks of pregnancy. Where applicable, gestational age was corrected by neonatal assessment based on the Dubowitz scoring system [6] for healthy infants, or the Farr scoring system [7] where infants required intensive care. Neither of these scoring systems are birth weight-dependent [ 11. Maternal age was recorded as the number of completed years at booking. Maternal height was measured in cm and weight at booking in kg. Parity was
of Gynecology
& Obstetks
57 (1997) 267-271
defined as the number of previous pregnancies with a gestation of 24 or more weeks. Immigrant status was attributed to women who had lived in Hong Kong for 10 years or less, and these were excluded. Maternal smoking was not analyzed as less than 1% of women delivered in the unit admitted to smoking [l]. 2.1. Statistical methods The data were analyzed in 2 groups representing the extremes of a decade. The first group consisted of all babies born between 1985-86, while the second included deliveries in 1995-96. For each group, the frequency distributions of birth weight, maternal age, gestational age at delivery, parity and fetal sex were determined (Table 1). Differences in means between groups for birth weight, maternal age, maternal weight at booking, maternal height and gestational age at delivery were calculated using Student’s t-test. Birth weight was further subdivided into three groups, I 1500 g, I 2500 g, and 2 4000 g. Differences in proportions between groups were assessed using the chi-square test. In order to adjust birth weight for maternal characteristics, multiple regression analysis was employed (‘enter all’ method) on the pooled data from the two groups. Independent variables included in the analysis were maternal height, maternal weight at booking, sex of the baby, parity and time group (i.e. group 1 or group 2). Gestational age was not included, for reasons given in Section 4. Categorical variables were transformed Table 1 Changes in gestational age
Mean (SD.) Number (%) post-term: 2 42 weeks Number (%) pre-term: < 37 weeks *P **P
Group 1 (1985-1986)
Group 2 (1995-1996)
39.4 (1.9) 992 (9.4)
38.9 (2.0)* 386 (4.9r**
264 (5.8)
240 (6.9r*
< 0.001 by Student’s t-test. < 0.001 by Chi-square test.
GM
Brieger et al. /International
Journal
into dummy variables prior to entry as follows. Multiparas were coded as 1, nulliparae 0. Male sex was entered as 1, female 0. Group 2 is coded as 1, group 1 is the reference (0). Cases with missing data were excluded from the analysis. To determine the factors related to the incidence of low birth weight and very low birth weight, logistic regression analysis was carried out. Independent variables included were maternal height, maternal weight at booking, gestational age, sex of the baby, parity and time group (i.e. group 1 or group 2). All statistical tests were performed with SPSS for Windows (version 7.0, SPSS Inc., CA). 3. Results
A total of 10512 confinements were reviewed for the years 1985-86 (group 11, and 7857 for the years 1995-96 (group 2). Data was missing in 17 cases, and there were no out-of-range values. Mean gestational age at delivery showed a significant decrease over the 10 years. The percentage of post-term pregnancies decreased from 9.5% to 4.8%, while preterm deliveries increased slightly from 5.8% to 6.9%. There was a significant increase in maternal Table 2 Maternal
and pregnancy
characteristics
Group 1 (1985-1986) Height (cm) Mean (SD.) Booking weight Mean (S.D.) Age (years) Mean (S.D.) BMI (kg/m*) mean (SD.) Parity (%I: Para 0 Para 1 Para 2 + Sex ratio (% females) “Student’s bChi-square
t-test. test.
Group 2 (1995-1996)
Significance: P-value
155.8 (5.3)
157.5 (5.5)
< O.OOOld
56.6 (8.7)
57.2 (8.9)
< 0.0001”
27.6 (4.5)
29.7 (5.0)
< 0.000 1”
23.3 (3.4)
23.1 (3.4)
< O.OOO1d
55.9 26.1 18.0
45.5 37.9 16.6
< O.OOO1b
49.5
47.9
0.036
of Gynecologv
& Obstettics
57 (1997) 267-271
269
height, weight at booking, and maternal age, whereas the body-mass index was reduced slightly (Table 2). Important changes in the distribution of parity and fetal sex were noted between the two groups. Parity increased significantly, with the percentage of parous women rising from 44.1% to 55.6%. The sex ratio changed significantly, so that the percentage of female infants decreased from 49.5% to 47.9%. There was no significant difference in mean birth weights between the two groups. However, when birth weight was controlled for sex, parity, maternal height and weight in the multiple regression there was a small but significant increase over time (Table 3), by 15 g. A significant increase in the number of babies weighing 5 1500 g was noted, whilst there was no change in the groups < 2500 g and in those L 4000 g. Logistic regression analysis on the factors related to very low birth weight and low birth weight showed that gestational age was the only significant variable; group membership was not significant. 4. Discussion
Birth weight is recognised as a global indicator of community health [8], and it has been shown to increase over time in developing countries [9]. This is speculatively attributed to improved environmental conditions [5,9]. Average birth weight in Hong Kong Chinese was reported in 1964 as Table 3 Changes in birth
weight Group 1 (1985-1986)
(kg)
Birth weight (g): Mean (S.D.) 3215 (465) Birth weight (g): Median (skew) 3200 ( - 0.38) Number (%) < 1500 g 51(0.49) Number (%) < 2500 g 630 (6.0) Number (%) > 4000 g 489 (4.7) “Chi-square test. hStudent’s t-test. ‘Kolgomorov-Smirnov distributions.
Group 2 (199551996)
3211 (499) 3200 75 517 381
test for difference
( - 0.81) (0.95) (6.6) (4.8)
P-value
0.57” < 0.001’ < O.OOO1b 0.109b 0.5sh
in the skew of the
270
G.M.
Brieger et al. /International
Journal
3110 g. Later, data comparing similar Hong Kong Chinese groups over a 12-year interval to 1984 found birth weight to have increased significantly at term from a mean birth weight at 40 weeks of 3130 g to 3237 g [2-41. The increase was confirmed in other series, and attributed in part to the rapid changes in socioeconomic and nutritional state [l]. Our findings suggest that over the last 10 years, mean birth weight has remained constant, whilst it is recognised that the socioeconomic and nutritional states have continued to change [lo]. We confirmed that both maternal height and weight, indicators of nutritional well-being [5], have continued to increase significantly over time (Table 2). This has occurred with a slight reduction in obesity, the BMI being significantly decreased. Birth weight is known to increase with gestational age, maternal weight, height, parity, male sex fetus, as well as increased maternal age [ill. Of these, gestational age has been shown to be the strongest variable. The changes in pregnancy characteristics that we have described, with the exception of gestational age, would have favored an increase in birth weight. So would a possible increase in the incidence of gestational diabetes [12] due to dietary changes. Multiple regression analysis allowed us to assess changes in birth weight while adjusting for sex and maternal characteristics; the results suggested an increase of only 15 g over the period studied (Table 4). This may be due to changes in cigarette consumption, as the proportion of women smokers has decreased over the last decade [13]. It therefore raises the possibility that the maximum mean birth weight has been reached for Hong Kong Chinese, despite continuing nutritional and socioeconomic growth. The significant increase in the very low birth weight group is cause for concern, as these babies have the highest rates of mortality and long-term morbidity. Whilst the numbers are not great enough to skew the distribution, and the quartile values as well as the median remain unchanged, it does suggest that the overall reproductive performance of Hong Kong women has deteriorated. The increase in assisted reproduction (although multiple pregnancy was excluded), and the in-
of Gynecology
& Obstetrics
57 (1997) 267-271
Table 4 Birth weight in relation to maternal characteristics: multiple regression analysis Variable
Coefficient
Standard error
T
P-value
Constant Age Height Weight Parity Sex (male) Group
- 3594.3 4.2 6.6 10.5 72.0 105.4 15.3
104.9 0.7 0.6 0.4 6.4 5.8 6.1
- 34.3 6.4 11.4 29.4 11.3 18.3 2.5
0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0120
Dummy variables are coded as follows: multiparas (11, nulliparae (0); male (l), female (0). Group 2 is coded as (l), group 1 is the reference (0).
Note.
crease in maternal age may account for some of the change. The increased proportion of male infants delivering in our unit over the decade was an unexpected finding. These changes are strongly suggestive of extraneous factor(s) operating during the last decade. Changes in the sex ratio attributable to increased exposure to alcohol and lead, as well as changes due to increased socioeconomic status have been described elsewhere [14,15]. Furthermore, feticide on the basis of sex-selection and the one-child policy have been described in mainland China, all of which may be factors in the population studied. Immigration from mainland China is unlikely to account for the change in mean birth weight and sex ratio, as only those women who had been resident in Hong Kong for more than 10 years were included in the analysis. The observed changes in the gestational age distribution, more preterm and less post-term pregnancies, are typically seen during a period of transition from dating by the last menstrual period to dating by ultrasound [16]. Over the time interval studied, ultrasound equipment has been used increasingly to assist in gestational age assignment. Currently, ultrasound examination at booking is reserved for those whose last menstrual period dates are unknown, uncertain or inconsistent with clinical examination. The apparent changes noted in the gestational age distribution are likely to be confounded by the method of gestational age assignment. Hence this parameter
GM
Brieger
et al. /International
Journal
was not included in the regression analysis, in contrast to the previous studies [l-3]. Analyses of similar birth weight data in Western countries have shown trends opposite to those noted in Hong Kong. When data was reviewed up to 1989 in the United States, England and Wales there had been a steady increase in the median birth weight which was attributed in part to a reduction in maternal smoking, and improved nutrition [ll]. It was also concluded that the shift towards higher birth weight should continue in Western countries. It appears that birth weight has reached a plateau in Hong Kong despite a continuing increase in the regions’ socioeconomic status, and evidence of improved nutritional well-being [17]. The analysis of this data enables an accelerated view due to the Territories rapid development, and the small but significant decrease in adjusted mean birth weight was indeed an unexpected finding. Similarly the change in sex ratio is suggestive of an unrecognized influence on this population. References ill PI
Dl [41
Rogers MS, Wong FWS, Chang AMZ. Determinants of birth-weight in the New Territories of Hong Kong. Aust NZ J Obstet Gynaecol 1987; 27: 314-319. Woo JSK, Li DFH, Ma HK. Intrauterine growth standards for Hong Kong Chinese. Aust NZ J Obstet Gynaecol 1986; 26: 54-58. Ip HMH. Intrauterine growth in Hong Kong Chinese. Biol Neonate 1978; 33: 253-263. Chun D, Kan PS, Ho HC, Wong CP. The Hong Kong Chinese female boney pelvis and its influence on labour: a radiographic and clinical study of 1005 women. J Obstet Gynaecol Br Commonw 1964; 71: 556-564.
of Gynecology
& Obstettics
57 (1997)
267-271
171
[A
Gruenwald P, Funakawa H, Mitani S, Nishimura T, Takeuchi S. Influence of environmental factors on foetal growth in man. Lancet 1967; 1: 1026-1028.
[hl
Dubowitz L. Assessment born: a practical scoring 44: 782.
[71
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