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New birth weight nomograms for twin gestation on the basis of accurate gestational age
New birth weight nomograms for twin gestation on the basis of accurate gestational age Shlomo B. Cohen, MD, Mordechai Dulitzky, MD, Shlomo Lipitz, MD, Shlomo Mashiach, MD, and Eyal Schiff, MD Tel Hashomer, Israel OBJECTIVE: Our purpose was to establish new nomograms for the birth weight of twins on the basis of accurate methods to validate gestational age. STUDY DESIGN: The medical records of 1632 consecutive twin gestations delivered between 1984 and 1996 were reviewed. Only pregnancies induced by ovulation induction techniques or that were measured ultrasonographically for crown-rump length during the first trimester were included. Excluded were those whose fetuses (one or both) were stillborn, or if the mother smoked, had a significant chronic illness, or was prescribed any regular medications. The study comprised 520 twin pregnancies at 28 to 41 gestational weeks at delivery. RESULTS; The median and 10th and 90th percentile birth weight curves were calculated for the studied twins and plotted against previously reported singleton nomograms. Fetuses of twin pregnancies were found to be growth restricted in comparison with previously reported singletons throughout the thircl trimester. This trend became more evident after the thirty-fourth to thirty-sixth weeks. CONCLUSIONS: We recommend these novel birth weight nomograms for clinical use in the management of twin pregnancies. (Am J Obstet Gynecol 1997;177:1 t01-4.)
Key words: Twins, birth weight n o m o g r a m s , growth restriction
T h e increasing use of assisted reproductive techniques in the past decade has increased the n u m b e r of twin pregnancies. ~ C o m p a r e d with singletons, twins are fivefold m o r e likely to be b o r n before term, and in the group of p r e t e r m births twins are m o r e than twice as likely to be in the g r o u p of low birth weight neonates. 2' s T h e perinatal mortality rate in twin pregnancies is four to five times h i g h e r than in singletons, < 5 with a relative risk of 6.6 for dying within the first y e a r ) Nowadays perinatologists are m o r e frequently exposed to twin gestations in which the growth of one or both fetuses is estimated as being restricted. In the past some authors p r o p o s e d the use of singleton standards to evaluate the intrauterine growth of twins, 6' 7 whereas others showed a clear trend of growth restriction in twins c o m p a r e d with standards of singletons, s Nevertheless, there is no broad consensus as to w h e t h e r the usage of singleton standards to evaluate the intrauterine growth of twins is appropriate, and some authors have advised d e v e l o p m e n t of twin-specific growth curves. `< 9-11 Data available in the current literature with respect to twin-specific growth curves seem to From the~ Buchmann Co,necology and Maternity Center, Chaim Sheba Medical Cente*, and the Sackler Faculty of Medicine, Tel Aviv University. Presented at the Seventeenth Annual Meeting of the Society of Perinatal Obstetricians, Anaheim, California, January 20-25, I997. Reprint requests: Eyal Schiff, MD, Department of Obstetrics and Gynecology, Chaim Sheba Medical Center, Tel Hashomer 52621, Israel. Copyright © 1997 by Mosby-Year Book, Inc. 0002-9378/97 $5.00 + 0 6/6/84379
lack accuracy in d e t e r m i n i n g gestational age, which is eventually critical for establishing accurate curves. T h e current study strived to present new intrauterine growth curves of twins on the basis of an accurate gestational age. Material and m e t h o d s
Between January 1984 and January 1996, 1632 w o m e n were admitted to our medical center for twin delivery at ->25 c o m p l e t e d weeks of gestation. T h e i r medical records were reviewed for e n r o l l m e n t into the study group according to the following inclusion criteria: (1) pregnancies that were i n d u c e d by ovulation induction techniques and therefore the date of ovulation could be estimated or (2) pregnancies where ultrasonographic m e a s u r e m e n t of fetal crown-rump length was p e r f o r m e d in the first trimester. Eight h u n d r e d sixteen pairs of twins who fulfilled these criteria were identified. F r o m this g r o u p we further excluded all the following cases: (1) pregnancies that resulted in o n e or both fetuses being stillborn or with major anomalies, (2) existence of significant chronic m a t e r n a l disease (cardiac, renal, respiratory), and (3) history of any drug regularly prescribed, smoking, or substance (including alcohol) abuse during the index pregnancy. With use of these criteria, a total of 296 pairs of twins were excluded. H e n c e the study group comprised 540 pairs of live-born twins. All of the population attending our medical center during the study p e r i o d was white, the majority ( > 9 9 % ) 1101
1102
C o h e n et al.
November 1997 Am J Obstet Gynecol
4000
3500
3000
2500
.E 2000
1500
1000
"D"SING LETONS-90% 28
29
30
31
32
33
34
35
36
37
38
39
_~
40
41
GESTATIONAL AGE IN WEEKS
Fig. 1. Israeli twins versus singleton (U.S.) birth weight curves.
T a b l e I. Twin birth weight percentiles in study group
Week 28 29 30 31 32 33 34 35 36 37 38 39 40 41
l Oth Percentile
50th Percentile
90th Percentile
675 705 945 1120 1245 1330 1430 1655 1865 2045 2130 2315 2215 2180
995 1145 1300 1445 1580 1750 1905 2165 2275 2430 2565 2680 2810 2685
I495 1495 1610 1795 1825 2030 2220 2590 2670 2895 3000 3160 3195 3205
Jewish (approximately half Ashkenazi and half Sephardic descent). Gestational age at delivery was d e t e r m i n e d by the ovulation date if available or, if n o t available, by crown-rump length measurement. In n o n e of those cases in which both crown-rump length and ovulation date were known was there a difference in gestational age of > 6 days between the two m o d e s of calculation. T h e newborns were g r o u p e d according to gestational age, which was r e c o r d e d in weeks plus days from 25 to 42 weeks (26 weeks 1 day to 27 weeks 0 days considered as 27 weeks). The m e d i a n and 10th and 90th percentile birth weight for each gestational week were calculated with use of standard statistical software (StatView, Abacus Concepts, Berkeley, Calif.). Curves were then plotted for the general population and for each g e n d e r separately. These
I
No. 14 16 26 24 30 32 46 78 134 156 244 148 70 22
curves were again plotted against previously r e p o r t e d singleton nomograms. Results
Table I presents the 10th, 50th, and 90th percentiles of the birth weights of the 1040 twin newborns included in the study. (During statistical analysis of the data a minim u m of 14 neonates for each week was r e q u i r e d to establish the nomograms. Thus 40 additional neonates of gestational ages 25 to 27 weeks and 42 weeks were excluded.) Figs. 1 and 2 present the growth curves as g e n e r a t e d f r o m this study for twins plotted against singleton birth weight n o m o g r a m s previously r e p o r t e d in the U n i t e d States 12 and in the local Israeli population, 13 respectively. Fig. 3 presents birth weight curves for twin neonates
Cohen et al.
Volume 177, Number 5
1103
AmJ Obstet GynecoI
'°°°1 3500
T
3000 I
..~_....---',<--
x
2500 ! ¢ 2000
., 1000 I _ _ _ + /
, ~,~-~0% --~'SINGLETONS-10%
500
~SINGLETONS-50% '-Q'SINGLETONS-90%
(]~ 25
I
29
--4---
30
J
31
~
32
i
I
;
33
34
35
Gestational
age
~
36
I
I
I
37
38
39
-
I
40
- -
41
in weeks
Fig. 2. Israeli twins versus singleton (Israel) birth weight curves. 3500 i
2000
10001
--MAL.0%
/ ~ " ~ f
--.tr FEMALF---I(]%
500 "
--o---FEMALE-50%
I
,,-o.-FEMALE-90%
/
28
29
30
31
32
33
34
Gestational
age
35 36 in weeks
37
38
39
40
41
Fig. 3. Male versus female Israeli twins. divided according to gender. O n average, twin male infants weighed 4.0% +_ 3.6% (83,2 gm _+ 66 gin) m o r e than female infants t h r o u g h o u t the tested gestational weeks.
Comment In the current study we established new birth weight nomograms for twins on the basis of accurate gestational age. Our strict criteria for validation of gestational age (e.g., d o c u m e n t e d ovulation date or crown-rump length measurement in the first trimester that is precise to _+5 gestational days)14 has left us with a population not previously
investigated by others for the establishment of such nomograms. Examples for the inaccurate determination of gestational age previously published are as follows. Luke et al.ll established nomograms based on gestational age calculated from the last menstrual period. This m e t h o d is problematic because only two thirds of adult women have cycles lasting 21 to 35 days 15 and many women tend to forget the exact date of the last menstruation. Furthermore, many of the twin gestations today are a result of ovulation induction techniques frequently performed because of cycle irregularities. Rydhstrom 1° in 60% of his study group used an ultrasonographic estimation of the fetal age in the second
\,
1104
Cohen et al.
trimester where the possible error is +10 days 14 for biparietal diameter. Our study compared the established nomograms with American singleton nomogrmns routinely used in our medical center and in many other medical centers around the world a2 and with the Israefi local population singleton nomograms presented by others. 13 Our findings show that from 28 weeks of gestation twins are growth restricted compared with American singletons (Fig. 1), with increase in the difference between the curves after the thirty-fourth week (where the 90th percentile for twins almost merges with the 50th percentile curve of singletons). C o m p a r e d with the Israeli singleton population (Fig. 2), a similar t r e n d is m a i n t a i n e d as above, b e c o m i n g m o r e evident after the thirty-seventh week. Mckeown and Record 16 stated similar growth until 27 weeks for twins and singletons and then a slow growth rate for twins f r o m the thirtieth week. Williams et al.s f o u n d that the difference o c c u r r e d at 26 weeks, with peak velocity at 31 weeks. Luke et al. la indicated a growth difference only at 36 weeks, and Naeye et a l . 17 detected differences in the thirty-third week. O u r data, based on precise gestational age determinations available starting at the twenty-eighth week, clearly demonstrate a significant relative growth restriction of twins c o m p a r e d with singletons t h r o u g h o u t the third trimester, with an i n c r e m e n t in the difference at the last 4 to 6 weeks of pregnancy. This difference may also be e n h a n c e d by the fact that pregnancies associated with risk factors for growth restriction (smoking, chronic illness) were e x c l u d e d from our twin group. Only anencephalus, hydrocephalus, hydrops, and maternal diabetes were e x c l u d e d by L u b c h e n c o et al. ~2 w h e n r e p o r t i n g singleton nomograms. All the o t h e r risk factors were n o t e x c l u d e d from the Israeli study. ~3 Nevertheless, because our major objective of the current study was to establish accurate twin-specific curves for the clinician and n o t to explore the differences between singleton and twin n o m o g r a m s , we o p t e d to exclude these cases. In our study the differences between twin males and females were, on average, 83.2 gm + 66 g m (4.0% -+ 3.6% for each week). These findings are in accordance with those of other investigators of twins, I°' n who reported a 92 gm and 42 gm m e a n difference in favor of male twins. For singletons, Lubchenco et al.12 reported an approximate difference of 100 gm, and Leiberman et al. ~3 a 121 gm difference, also both in favor of males, We elected to conduct our study on the lbasis of actual birth weight rather than ultrasonographic estimated fetal weight to make the nomograms available for the clinician, who should not rely on estimated weights only when facing a dilemma concerning the m a n a g e m e n t of twin pregnancy with suspected ultrasonographic growth restriction. A m o n g the studied pairs of twins, 69 (12.7 %) were found to be severely discordant (>25% difference between birth
November 1997 Am J Obstet Gynecol
weight within a pair), and an additional 113 pairs (21.0%) had mild discordancy (>15% and <-25% difference). T h e s e pairs of d i s c o r d a n t twins were n o t e x c l u d e d f r o m the analysis because they c o n t r i b u t e to the real birth w e i g h t distribution o f the twin p o p u l a t i o n . T h e philosophical debate of how to define intrauterine growth restriction remains unsolved with regard to twin pregnancy. T h e rationale of using singleton curves for twins is that, regardless of the causes of growth restriction, these fetuses that are below specific weight for gestational age may be at increased risk in the intrauterine environment. The above hypothesis may be c o n f i r m e d by a future study in which the perinatal o u t c o m e of twins will be investigated by c o m p a r i n g those < 1 0 t h percentile for singletons with those between that curve and the twinspecific 10th percentile curve established in our study. REFERENCES
1. National Center for Health Statistics. Advance report of final natality statistics, 1989. Hyattsville (MD): Public Health Service; 1991. Monthly Vital Statistics Report, Volume 40, No. 8, Supplement. 2. Luke B, Minogue J, Witter F. The role of fetal growth restriction and gestational age on length of hospital stay in twin infants. Obstet Gynecol 1993;81:949-53. 3. Luke B, Keith LG. The contribution of singletons, twins and triplets to low birth weight, infant mortality" and handicap in the United States. J Reprod Med 1992;37:661-6. 4. Herruzo AJ, Martinez L, Biel E, Robles R, Resales MA, Miranda JA. Perinatal morbidity and mortality in twin pregnancies. IntJ Gynecol Obstet 1991;36:17-22. 5. Fliegner JR, Eggers TR. The relationship between gestational age and birth-weight in re,in pregnancy. Aust N Z J Obstet Gynaecol 1984;24:192-7. 6. Bronsteen R, Goyert G, Bottoms S. Classification of twins and neonatal morbidity. Obstet Gynecol 1989;74:98-101. 7. Nellson JP. Fetal growth in twin pregnancies. Acta Genet Med Gemellol 1989;37:409-16. 8. Williams RL, Creasy RK, Cunningham GC, Hawes WE, Norris FD, Tashiro M. Petal growth and perinatal viability in California. Obstet Gynecol 1989;59:624-32. 9. Leroy B, Lefort F, Neveu P, Risse RJ, Trevise P, Jeny R. Intrauterine growth charts for twins fetuses, Acta Genet Med Gemellol 1982;31:199-206. 10. Rydhstrom H. A birthweight-for-gestation standard based on 4737 twins born in Sweden between 1983 and 1985. Acta Obstet Gynecol Scand 1992;71:506-11. 11. Luke B, Witter FR, Abbey H, Feng T, Namnoum AB, Paige DM, et al. Gestational age-specific birthweights of twins versus singletons. Acta Genet Med Gemellol 1991;40:69-76. 12. Lubchenco LO, Hansman C, Dressier M, Boyd E. Intrauterine growth as estimated from liveborn birth-weight data at 24 to 42 weeks of gestation. Pediatrics 1963;39:793-800. 13. Leiberman RJ, Fraser D, Weitzman S, Glezerman M. Birthweight curves in southern Israel populations. Isr Med Sci 1993;29:198-203. 14. Scott JR, Disaia PJ, Hammond CB, Spellacy WN. Danforth's obstetrics and gynecology. 7th edition. Philadelphia: JB Lippincott; 1994. 15. Berek SJ, Adashi EY, Hillard PA. Novak's gynecology. 12th edition. Baltimore: Williams & Wilkins; 1996. 16. Mckeown T, Record RG. Observations on foetal growth in multiple pregnancy in man. J Endocrinol 1952;8:386-401. 17. Naeye RL, Benirschke K, HagstromJWC, Marcus CC. Intrauterine growth of twins as estimated from liveborn birthweight data. Pediatrics 1966;37:35-9.
Key words: Twins, birth weight n o m o g r a m s , growth restriction
T h e increasing use of assisted reproductive techniques in the past decade has increased the n u m b e r of twin pregnancies. ~ C o m p a r e d with singletons, twins are fivefold m o r e likely to be b o r n before term, and in the group of p r e t e r m births twins are m o r e than twice as likely to be in the g r o u p of low birth weight neonates. 2' s T h e perinatal mortality rate in twin pregnancies is four to five times h i g h e r than in singletons, < 5 with a relative risk of 6.6 for dying within the first y e a r ) Nowadays perinatologists are m o r e frequently exposed to twin gestations in which the growth of one or both fetuses is estimated as being restricted. In the past some authors p r o p o s e d the use of singleton standards to evaluate the intrauterine growth of twins, 6' 7 whereas others showed a clear trend of growth restriction in twins c o m p a r e d with standards of singletons, s Nevertheless, there is no broad consensus as to w h e t h e r the usage of singleton standards to evaluate the intrauterine growth of twins is appropriate, and some authors have advised d e v e l o p m e n t of twin-specific growth curves. `< 9-11 Data available in the current literature with respect to twin-specific growth curves seem to From the~ Buchmann Co,necology and Maternity Center, Chaim Sheba Medical Cente*, and the Sackler Faculty of Medicine, Tel Aviv University. Presented at the Seventeenth Annual Meeting of the Society of Perinatal Obstetricians, Anaheim, California, January 20-25, I997. Reprint requests: Eyal Schiff, MD, Department of Obstetrics and Gynecology, Chaim Sheba Medical Center, Tel Hashomer 52621, Israel. Copyright © 1997 by Mosby-Year Book, Inc. 0002-9378/97 $5.00 + 0 6/6/84379
lack accuracy in d e t e r m i n i n g gestational age, which is eventually critical for establishing accurate curves. T h e current study strived to present new intrauterine growth curves of twins on the basis of an accurate gestational age. Material and m e t h o d s
Between January 1984 and January 1996, 1632 w o m e n were admitted to our medical center for twin delivery at ->25 c o m p l e t e d weeks of gestation. T h e i r medical records were reviewed for e n r o l l m e n t into the study group according to the following inclusion criteria: (1) pregnancies that were i n d u c e d by ovulation induction techniques and therefore the date of ovulation could be estimated or (2) pregnancies where ultrasonographic m e a s u r e m e n t of fetal crown-rump length was p e r f o r m e d in the first trimester. Eight h u n d r e d sixteen pairs of twins who fulfilled these criteria were identified. F r o m this g r o u p we further excluded all the following cases: (1) pregnancies that resulted in o n e or both fetuses being stillborn or with major anomalies, (2) existence of significant chronic m a t e r n a l disease (cardiac, renal, respiratory), and (3) history of any drug regularly prescribed, smoking, or substance (including alcohol) abuse during the index pregnancy. With use of these criteria, a total of 296 pairs of twins were excluded. H e n c e the study group comprised 540 pairs of live-born twins. All of the population attending our medical center during the study p e r i o d was white, the majority ( > 9 9 % ) 1101
1102
C o h e n et al.
November 1997 Am J Obstet Gynecol
4000
3500
3000
2500
.E 2000
1500
1000
"D"SING LETONS-90% 28
29
30
31
32
33
34
35
36
37
38
39
_~
40
41
GESTATIONAL AGE IN WEEKS
Fig. 1. Israeli twins versus singleton (U.S.) birth weight curves.
T a b l e I. Twin birth weight percentiles in study group
Week 28 29 30 31 32 33 34 35 36 37 38 39 40 41
l Oth Percentile
50th Percentile
90th Percentile
675 705 945 1120 1245 1330 1430 1655 1865 2045 2130 2315 2215 2180
995 1145 1300 1445 1580 1750 1905 2165 2275 2430 2565 2680 2810 2685
I495 1495 1610 1795 1825 2030 2220 2590 2670 2895 3000 3160 3195 3205
Jewish (approximately half Ashkenazi and half Sephardic descent). Gestational age at delivery was d e t e r m i n e d by the ovulation date if available or, if n o t available, by crown-rump length measurement. In n o n e of those cases in which both crown-rump length and ovulation date were known was there a difference in gestational age of > 6 days between the two m o d e s of calculation. T h e newborns were g r o u p e d according to gestational age, which was r e c o r d e d in weeks plus days from 25 to 42 weeks (26 weeks 1 day to 27 weeks 0 days considered as 27 weeks). The m e d i a n and 10th and 90th percentile birth weight for each gestational week were calculated with use of standard statistical software (StatView, Abacus Concepts, Berkeley, Calif.). Curves were then plotted for the general population and for each g e n d e r separately. These
I
No. 14 16 26 24 30 32 46 78 134 156 244 148 70 22
curves were again plotted against previously r e p o r t e d singleton nomograms. Results
Table I presents the 10th, 50th, and 90th percentiles of the birth weights of the 1040 twin newborns included in the study. (During statistical analysis of the data a minim u m of 14 neonates for each week was r e q u i r e d to establish the nomograms. Thus 40 additional neonates of gestational ages 25 to 27 weeks and 42 weeks were excluded.) Figs. 1 and 2 present the growth curves as g e n e r a t e d f r o m this study for twins plotted against singleton birth weight n o m o g r a m s previously r e p o r t e d in the U n i t e d States 12 and in the local Israeli population, 13 respectively. Fig. 3 presents birth weight curves for twin neonates
Cohen et al.
Volume 177, Number 5
1103
AmJ Obstet GynecoI
'°°°1 3500
T
3000 I
..~_....---',<--
x
2500 ! ¢ 2000
., 1000 I _ _ _ + /
, ~,~-~0% --~'SINGLETONS-10%
500
~SINGLETONS-50% '-Q'SINGLETONS-90%
(]~ 25
I
29
--4---
30
J
31
~
32
i
I
;
33
34
35
Gestational
age
~
36
I
I
I
37
38
39
-
I
40
- -
41
in weeks
Fig. 2. Israeli twins versus singleton (Israel) birth weight curves. 3500 i
2000
10001
--MAL.0%
/ ~ " ~ f
--.tr FEMALF---I(]%
500 "
--o---FEMALE-50%
I
,,-o.-FEMALE-90%
/
28
29
30
31
32
33
34
Gestational
age
35 36 in weeks
37
38
39
40
41
Fig. 3. Male versus female Israeli twins. divided according to gender. O n average, twin male infants weighed 4.0% +_ 3.6% (83,2 gm _+ 66 gin) m o r e than female infants t h r o u g h o u t the tested gestational weeks.
Comment In the current study we established new birth weight nomograms for twins on the basis of accurate gestational age. Our strict criteria for validation of gestational age (e.g., d o c u m e n t e d ovulation date or crown-rump length measurement in the first trimester that is precise to _+5 gestational days)14 has left us with a population not previously
investigated by others for the establishment of such nomograms. Examples for the inaccurate determination of gestational age previously published are as follows. Luke et al.ll established nomograms based on gestational age calculated from the last menstrual period. This m e t h o d is problematic because only two thirds of adult women have cycles lasting 21 to 35 days 15 and many women tend to forget the exact date of the last menstruation. Furthermore, many of the twin gestations today are a result of ovulation induction techniques frequently performed because of cycle irregularities. Rydhstrom 1° in 60% of his study group used an ultrasonographic estimation of the fetal age in the second
\,
1104
Cohen et al.
trimester where the possible error is +10 days 14 for biparietal diameter. Our study compared the established nomograms with American singleton nomogrmns routinely used in our medical center and in many other medical centers around the world a2 and with the Israefi local population singleton nomograms presented by others. 13 Our findings show that from 28 weeks of gestation twins are growth restricted compared with American singletons (Fig. 1), with increase in the difference between the curves after the thirty-fourth week (where the 90th percentile for twins almost merges with the 50th percentile curve of singletons). C o m p a r e d with the Israeli singleton population (Fig. 2), a similar t r e n d is m a i n t a i n e d as above, b e c o m i n g m o r e evident after the thirty-seventh week. Mckeown and Record 16 stated similar growth until 27 weeks for twins and singletons and then a slow growth rate for twins f r o m the thirtieth week. Williams et al.s f o u n d that the difference o c c u r r e d at 26 weeks, with peak velocity at 31 weeks. Luke et al. la indicated a growth difference only at 36 weeks, and Naeye et a l . 17 detected differences in the thirty-third week. O u r data, based on precise gestational age determinations available starting at the twenty-eighth week, clearly demonstrate a significant relative growth restriction of twins c o m p a r e d with singletons t h r o u g h o u t the third trimester, with an i n c r e m e n t in the difference at the last 4 to 6 weeks of pregnancy. This difference may also be e n h a n c e d by the fact that pregnancies associated with risk factors for growth restriction (smoking, chronic illness) were e x c l u d e d from our twin group. Only anencephalus, hydrocephalus, hydrops, and maternal diabetes were e x c l u d e d by L u b c h e n c o et al. ~2 w h e n r e p o r t i n g singleton nomograms. All the o t h e r risk factors were n o t e x c l u d e d from the Israeli study. ~3 Nevertheless, because our major objective of the current study was to establish accurate twin-specific curves for the clinician and n o t to explore the differences between singleton and twin n o m o g r a m s , we o p t e d to exclude these cases. In our study the differences between twin males and females were, on average, 83.2 gm + 66 g m (4.0% -+ 3.6% for each week). These findings are in accordance with those of other investigators of twins, I°' n who reported a 92 gm and 42 gm m e a n difference in favor of male twins. For singletons, Lubchenco et al.12 reported an approximate difference of 100 gm, and Leiberman et al. ~3 a 121 gm difference, also both in favor of males, We elected to conduct our study on the lbasis of actual birth weight rather than ultrasonographic estimated fetal weight to make the nomograms available for the clinician, who should not rely on estimated weights only when facing a dilemma concerning the m a n a g e m e n t of twin pregnancy with suspected ultrasonographic growth restriction. A m o n g the studied pairs of twins, 69 (12.7 %) were found to be severely discordant (>25% difference between birth
November 1997 Am J Obstet Gynecol
weight within a pair), and an additional 113 pairs (21.0%) had mild discordancy (>15% and <-25% difference). T h e s e pairs of d i s c o r d a n t twins were n o t e x c l u d e d f r o m the analysis because they c o n t r i b u t e to the real birth w e i g h t distribution o f the twin p o p u l a t i o n . T h e philosophical debate of how to define intrauterine growth restriction remains unsolved with regard to twin pregnancy. T h e rationale of using singleton curves for twins is that, regardless of the causes of growth restriction, these fetuses that are below specific weight for gestational age may be at increased risk in the intrauterine environment. The above hypothesis may be c o n f i r m e d by a future study in which the perinatal o u t c o m e of twins will be investigated by c o m p a r i n g those < 1 0 t h percentile for singletons with those between that curve and the twinspecific 10th percentile curve established in our study. REFERENCES
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