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Comprehensive fetal ultrasonographic growth measurements in triplet gestations
Comprehensive fetal ultrasonographic growth measurements in triplet gestations John F. Rodis, MD, Lawrence Arky, MD, James F.X. Egan, MD, Adam F. Borgida, MD, Mauro V. Leo, MD, and Winston A. Campbell, MD Farmington, Connecticut OBJECTIVE: Our purpose was to create tables and graphs of ultrasonographically derived fetal growth parameters in longitudinally studied triplet gestations from a single center. STUDY DESIGN: All triplet pregnancies managed by our division from 1987 through 1998 were identified. All had first-trimester dating sonograms and complete obstetric sonograms obtained by means of 3.5- or 5.0MHz curvilinear transducers with freeze-freeze capability and on-screen calipers. Sonograms to assess fetal growth were obtained every 2 to 4 weeks, from 16 to 18 weeks’ gestation until delivery. Fetal parameters obtained with each sonogram included biparietal diameter; head circumference; bicerebellar diameter; abdominal circumference; femur, humerus, tibia, and fibula lengths; estimated fetal weight; and head circumference/abdominal circumference ratio. Regression analysis was performed with JMP and Cricket Graph software packages, and lines of best fit with 95% confidence intervals were generated. RESULTS: A total of 443 ultrasonographic examinations were performed for 33 triplet pregnancies (99 fetuses). Each had between 3 and 6 sonograms obtained, all between 16 and 35 weeks’ gestation. Scatterplots of each of the fetal growth parameters against gestational age were created with regression lines of best fit and 95% confidence intervals. All growth parameters were dependent on gestational age. CONCLUSION: A comprehensive set of fetal growth measurements in triplets from the United States is now available and can be used to assess longitudinal fetal growth. (Am J Obstet Gynecol 1999;181:1128-32.)
Key words: Triplets, intrauterine fetal growth, ultrasonography
The number of triplet gestations in the United States has risen dramatically in the past 15 years. From 1980 to 1995, the birth rate for triplets and other high-order multiple gestations has increased from 37 per 100,000 live births to 127.5 per 100,000 live births, an increase of 345%.1, 2 Fetal growth in triplets is best assessed by serial ultrasonography. Weissman et al3 demonstrated that triplet growth paralleled singleton growth curves until 28 weeks’ gestation and then slowed. Mordel et al 4 demonstrated a 1- to 3-week delay in fetal growth of triplets compared with that in singletons. However, a comprehensive set of ultrasonographically derived fetal growth curves in US triplets is lacking. Thus the purpose of our study was to create tables and graphs of a comprehensive set of ultrasonographically derived fetal growth parameters in triplet gestations from a single US center.
From the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, University of Connecticut Health Center. Presented at the Nineteenth Annual Meeting of the Society for MaternalFetal Medicine, San Francisco, California, January 18-23, 1999. Reprints not available from the authors. Copyright © 1999 by Mosby, Inc. 0002-9378/99 $8.00 + 0 6/6/101074
1128
Material and methods All triplet pregnancies managed by our maternal-fetal medicine division from 1987 through 1998 were identified and the charts reviewed. All patients had firsttrimester dating sonograms, and members of our division, using 3.5- and 5.0-MHz curvilinear transducers, performed a complete obstetric ultrasonographic examination. Data on all patients were collected prospectively after admission to our practice. No patients were excluded. No patient had undergone multifetal pregnancy reduction. Our standard protocol for triplets during this period included serial sonograms to assess fetal growth every 2 to 4 weeks, from 16 to 18 weeks’ gestation until delivery. At each ultrasonographic examination, efforts were made to accurately identify each fetus on the basis of gender, lie, presentation, and placental location. Fetal parameters obtained with each sonogram included biparietal diameter (BPD), head circumference (HC), bicerebellar diameter, abdominal circumference (AC), femur length (FL), humerus length, tibia length, and fibula length. Estimated fetal weights were calculated with either the formula of Shepard et al,5 with BPD and AC, or the formula of Hadlock et al,6 with FL and AC, or with both. When estimated fetal weight was derived by means of both calculations, the average of the 2 weights was used. Discordance in estimated fetal weights was cal-
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Volume 181, Number 5, Part 1 Am J Obstet Gynecol
Fig 1. Scatterplot of biparietal diameter by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 3. Scatterplot of bicerebellar diameter by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 2. Scatterplot of head circumference by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 4. Scatterplot of abdominal circumference by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
culated between triplets 1 and 2, 2 and 3, and 1 and 3 for each ultrasonographic examination by the following formula: [(Weight of larger fetus – Weight of smaller fetus)/Weight of larger fetus] × 100%. HC/AC ratios were derived. Linear regression analysis of each variable was performed with the least-squares model of regression. Growth curves were created for each of the parameters, and lines of best fit and 95% confidence intervals were created with the JMP (SAS Institute, Cary, NC) and Cricket Graph (Cricket Software, Malvern, Pa) statistical software packages. A table was subsequently created from the regression formulas that best fit the curves.
Results A total of 442 ultrasonographic examinations were performed on 33 triplet pregnancies, which included 99 fetuses. Each mother had between 3 and 5 sonograms obtained, all between 16 and 35 weeks’ gestation. All infants were alive at birth. Average gestational age at delivery was 31.5 ± 5.6 weeks, and mean birth weight was 1496 ± 928 g. No case of fetal-fetal transfusion was identified. No case of insulin-requiring gestational diabetes developed. Indications for delivery were preterm labor or premature rupture of membranes, or both, in 16 pregnancies, preeclampsia or atypical preeclampsia in 11, and elective delivery at 34 to 36 weeks in 6. Scatterplots with regres-
1130 Rodis et al
Fig 5. Scatterplot of femur length by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
November 1999 Am J Obstet Gynecol
Fig 7. Scatterplot of tibia length by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 8. Scatterplot of fibula length by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown. Fig 6. Scatterplot of humerus length by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
sion curves for BPD, HC, bicerebellar diameter, AC, FL, humerus length, tibia length, fibula length, HC/AC ratio, and estimated fetal weight against gestational age (GA) in weeks are shown in Figs 1 to 10. All growth parameters are dependent on gestational age, and all fit second-degree polynomial equations the best, with the exception of bicerebellar diameter and HC/AC ratio, both of which fit linear equations. The regression equations for each of these parameters, with the corresponding R2 values, are as follows: BPD = –3.6497 + 0.53925GA – 0.0057854GA2; therefore R 2 = 0.955
HC = –12.688 + 1.9045GA – 0.019621GA2; therefore R 2 = 0.955 Bicerebellar diameter = –0.75130 + 0.13803GA; therefore R 2 = 0.919 AC = –11.349 + 1.5950GA – 0.0013321GA2; therefore R 2 = 0.923 FL = –4.1078 + 0.46156GA – 0.0048778GA2; therefore R 2 = 0.931 Humerus length = –3.5696 + 0.43558GA – 0.0052198GA2; therefore R 2 = 0.912 Tibia length = –4.4504 + 0.47480GA – 0.0056732GA2; therefore R 2 = 0.927 Fibula length = –4.2287 + 0.44881GA – 0.0052252GA2; therefore R 2 = 0.942
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Fig 9. Scatterplot of estimated fetal weight by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 10. Scatterplot of HC/AC ratio by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Table I. Growth parameters in triplets generated from regression equations Weeks
BPD
HC
Bicerebellar diameter
Humerus length
FL
Tibia length
Fibula length
AC
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
3.5 3.9 4.2 4.5 4.8 5.1 5.4 5.7 6.0 6.2 6.5 6.7 6.9 7.1 7.3 7.5 7.7 7.8 8.0 8.1
12.8 14.1 15.3 16.5 17.6 18.7 19.8 20.8 21.8 22.7 23.6 24.5 25.3 26.1 26.8 27.5 28.1 28.7 29.2 29.7
1.5 1.6 1.7 1.9 2.0 2.2 2.3 2.5 2.6 2.7 2.8 3.0 3.1 3.2 3.4 3.5 3.6 3.8 3.9 4.0
2.1 2.3 2.6 2.8 3.1 3.3 3.5 3.7 3.9 4.1 4.2 4.4 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.2
2.0 2.3 2.6 2.9 3.2 3.5 3.7 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.5 5.7 5.8 5.9 6.0
1.7 2.0 2.3 2.5 2.8 3.1 3.3 3.5 3.7 3.9 4.1 4.2 4.4 4.5 4.7 4.8 4.9 4.9 5.0 5.1
1.6 1.9 2.2 2.4 2.7 2.9 3.2 3.4 3.6 3.7 3.9 4.1 4.2 4.3 4.4 4.6 4.6 4.7 4.8 4.8
10.7 11.9 13.1 14.2 15.3 16.4 17.4 18.4 19.4 20.3 21.2 22.1 22.9 23.7 24.5 25.2 25.9 26.6 27.2 27.8
Estimated fetal weight = 271.45 – 53.886GA + 0.0029413GA2; therefore R2 = 0.887 HC/AC ratio = 1.2976 – 0.0068615GA; therefore R2 = 0.221 Table I demonstrates the expected value of BPD, HC, bicerebellar diameter, humerus length, FL, tibia length, fibula length, AC, HC/AC ratio, and estimated fetal weight predicted by the regression equations at each week of gestation from 16 to 35 weeks. When estimated fetal weight calculated by BPD and AC was compared with estimated fetal weight calculated from FL and AC, no significant differences were noted. As such, we chose to use the average of the 2 methods, because one cannot always obtain all measurements in high-order multifetal
pregnancies. The percent discordance for estimated fetal weight is also dependent on gestational age (R2 = 0.059). The 95th percentile of the confidence interval for percent discordance is 25% at 18 weeks and increases to 32% by 32 weeks. Comment It is well established that fetal growth is best assessed on ultrasonographic measurements generated from the population of the individuals under study. When such data are not available, curves from a population similar to the one under study should be used. Such curves are readily available for singleton and twin gestations.7, 8 However, a comprehensive set of these parameters has
1132 Rodis et al
not been available from a cohort of triplet fetuses from the United States. It is also clear that fetal growth and birth weight in triplets do not follow growth curves of either singletons or twins, as demonstrated recently by Alexander et al,9 using data from the National Center for Health Statistics. By comparing birth weights of singletons born alive in 1995 with birth weights of twins and triplets born alive from 1991 to 1995, they demonstrated that fetal growth in triplets begins to deviate from that of singletons and twins at approximately 31 weeks, after which the birth weight was significantly lower at each gestational age. Triplet pregnancies are at very high risk of preterm birth for numerous reasons, in particular preterm labor and preeclampsia, as our patients demonstrated. Because one would have no way of knowing which triplet pregnancy is destined to result in which of these complications, stratifying the data by indication for delivery or excluding such patients from analysis would bias the sample and make it clinically less useful. We chose to include all patients in our analysis, regardless of the indication for delivery, because they were ascertained prospectively. Large data sets of growth measurements in triplets, with more measurements at each week of gestation, obtained from the local population would be ideal. However, because such data sets may not be available, we believe that the graphs and table gener-
November 1999 Am J Obstet Gynecol
ated from this comprehensive set of fetal growth measurements can be used to assess longitudinal fetal growth of triplets. REFERENCES
1. Ventura SJ, Martin JA, Clarke SC, Mathews TJ. Report of final natality statistics, 1995. Hyattsville (MD): National Center for Health Statistics; 1997 Vol 31(Suppl 2). Monthly Vital Statistics Report No.: 11. 2. Martin JA, MacDorman MF, Mathews TJ. Triplet births: trends and outcomes, 1971-1994. National Center for Health Statistics. Vital Health Stat 21 1997:55. 3. Weissman A, Jakobi P, Yoffe N, Zimmer EZ, Paldi E, Brandes JM. Sonographic growth measurements in triplet pregnancies. Obstet Gynecol 1990;75:324-8. 4. Mordel N, Laufer N, Zajicek G, Shalev Z, Levin A, Schenker JG, et al. Sonographic growth curves of triplet conceptions. Am J Perinatol 1993;10:239-42. 5. Shepard MJ, Richards VA, Berkowitz RL, Warsof SL, Hobbins JC. An evaluation of two equations for predicting fetal weight by ultrasound. Am J Obstet Gynecol 1982;142:47-54. 6. Hadlock FP, Harrist RB, Carpenter RJ, Deter RL, Park SK. Sonographic estimation of fetal weight: the value of femur length in addition to head and abdomen measurements. Radiology 1984;150:535-40. 7. Hadlock FP, Deter RL, Harrist RB, Park SK. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology 1984;152:497-501. 8. Rodis JF, Vintzileos AM, Campbell WA, Pinette MG, Nochimson DJ. Intrauterine fetal growth in concordant twin gestations. Am J Obstet Gynecol 1990;162:1025-9. 9. Alexander GR, Kogan M, Martin J, Papiernik E. What are the fetal growth patterns of singletons, twins, and triplets in the United States? Clin Obstet Gynecol 1998;41:114-25.
Key words: Triplets, intrauterine fetal growth, ultrasonography
The number of triplet gestations in the United States has risen dramatically in the past 15 years. From 1980 to 1995, the birth rate for triplets and other high-order multiple gestations has increased from 37 per 100,000 live births to 127.5 per 100,000 live births, an increase of 345%.1, 2 Fetal growth in triplets is best assessed by serial ultrasonography. Weissman et al3 demonstrated that triplet growth paralleled singleton growth curves until 28 weeks’ gestation and then slowed. Mordel et al 4 demonstrated a 1- to 3-week delay in fetal growth of triplets compared with that in singletons. However, a comprehensive set of ultrasonographically derived fetal growth curves in US triplets is lacking. Thus the purpose of our study was to create tables and graphs of a comprehensive set of ultrasonographically derived fetal growth parameters in triplet gestations from a single US center.
From the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, University of Connecticut Health Center. Presented at the Nineteenth Annual Meeting of the Society for MaternalFetal Medicine, San Francisco, California, January 18-23, 1999. Reprints not available from the authors. Copyright © 1999 by Mosby, Inc. 0002-9378/99 $8.00 + 0 6/6/101074
1128
Material and methods All triplet pregnancies managed by our maternal-fetal medicine division from 1987 through 1998 were identified and the charts reviewed. All patients had firsttrimester dating sonograms, and members of our division, using 3.5- and 5.0-MHz curvilinear transducers, performed a complete obstetric ultrasonographic examination. Data on all patients were collected prospectively after admission to our practice. No patients were excluded. No patient had undergone multifetal pregnancy reduction. Our standard protocol for triplets during this period included serial sonograms to assess fetal growth every 2 to 4 weeks, from 16 to 18 weeks’ gestation until delivery. At each ultrasonographic examination, efforts were made to accurately identify each fetus on the basis of gender, lie, presentation, and placental location. Fetal parameters obtained with each sonogram included biparietal diameter (BPD), head circumference (HC), bicerebellar diameter, abdominal circumference (AC), femur length (FL), humerus length, tibia length, and fibula length. Estimated fetal weights were calculated with either the formula of Shepard et al,5 with BPD and AC, or the formula of Hadlock et al,6 with FL and AC, or with both. When estimated fetal weight was derived by means of both calculations, the average of the 2 weights was used. Discordance in estimated fetal weights was cal-
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Volume 181, Number 5, Part 1 Am J Obstet Gynecol
Fig 1. Scatterplot of biparietal diameter by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 3. Scatterplot of bicerebellar diameter by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 2. Scatterplot of head circumference by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 4. Scatterplot of abdominal circumference by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
culated between triplets 1 and 2, 2 and 3, and 1 and 3 for each ultrasonographic examination by the following formula: [(Weight of larger fetus – Weight of smaller fetus)/Weight of larger fetus] × 100%. HC/AC ratios were derived. Linear regression analysis of each variable was performed with the least-squares model of regression. Growth curves were created for each of the parameters, and lines of best fit and 95% confidence intervals were created with the JMP (SAS Institute, Cary, NC) and Cricket Graph (Cricket Software, Malvern, Pa) statistical software packages. A table was subsequently created from the regression formulas that best fit the curves.
Results A total of 442 ultrasonographic examinations were performed on 33 triplet pregnancies, which included 99 fetuses. Each mother had between 3 and 5 sonograms obtained, all between 16 and 35 weeks’ gestation. All infants were alive at birth. Average gestational age at delivery was 31.5 ± 5.6 weeks, and mean birth weight was 1496 ± 928 g. No case of fetal-fetal transfusion was identified. No case of insulin-requiring gestational diabetes developed. Indications for delivery were preterm labor or premature rupture of membranes, or both, in 16 pregnancies, preeclampsia or atypical preeclampsia in 11, and elective delivery at 34 to 36 weeks in 6. Scatterplots with regres-
1130 Rodis et al
Fig 5. Scatterplot of femur length by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
November 1999 Am J Obstet Gynecol
Fig 7. Scatterplot of tibia length by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 8. Scatterplot of fibula length by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown. Fig 6. Scatterplot of humerus length by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
sion curves for BPD, HC, bicerebellar diameter, AC, FL, humerus length, tibia length, fibula length, HC/AC ratio, and estimated fetal weight against gestational age (GA) in weeks are shown in Figs 1 to 10. All growth parameters are dependent on gestational age, and all fit second-degree polynomial equations the best, with the exception of bicerebellar diameter and HC/AC ratio, both of which fit linear equations. The regression equations for each of these parameters, with the corresponding R2 values, are as follows: BPD = –3.6497 + 0.53925GA – 0.0057854GA2; therefore R 2 = 0.955
HC = –12.688 + 1.9045GA – 0.019621GA2; therefore R 2 = 0.955 Bicerebellar diameter = –0.75130 + 0.13803GA; therefore R 2 = 0.919 AC = –11.349 + 1.5950GA – 0.0013321GA2; therefore R 2 = 0.923 FL = –4.1078 + 0.46156GA – 0.0048778GA2; therefore R 2 = 0.931 Humerus length = –3.5696 + 0.43558GA – 0.0052198GA2; therefore R 2 = 0.912 Tibia length = –4.4504 + 0.47480GA – 0.0056732GA2; therefore R 2 = 0.927 Fibula length = –4.2287 + 0.44881GA – 0.0052252GA2; therefore R 2 = 0.942
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Fig 9. Scatterplot of estimated fetal weight by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Fig 10. Scatterplot of HC/AC ratio by gestational weeks in triplets. Regression line and 5% and 95% confidence intervals are shown.
Table I. Growth parameters in triplets generated from regression equations Weeks
BPD
HC
Bicerebellar diameter
Humerus length
FL
Tibia length
Fibula length
AC
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
3.5 3.9 4.2 4.5 4.8 5.1 5.4 5.7 6.0 6.2 6.5 6.7 6.9 7.1 7.3 7.5 7.7 7.8 8.0 8.1
12.8 14.1 15.3 16.5 17.6 18.7 19.8 20.8 21.8 22.7 23.6 24.5 25.3 26.1 26.8 27.5 28.1 28.7 29.2 29.7
1.5 1.6 1.7 1.9 2.0 2.2 2.3 2.5 2.6 2.7 2.8 3.0 3.1 3.2 3.4 3.5 3.6 3.8 3.9 4.0
2.1 2.3 2.6 2.8 3.1 3.3 3.5 3.7 3.9 4.1 4.2 4.4 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.2
2.0 2.3 2.6 2.9 3.2 3.5 3.7 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.5 5.7 5.8 5.9 6.0
1.7 2.0 2.3 2.5 2.8 3.1 3.3 3.5 3.7 3.9 4.1 4.2 4.4 4.5 4.7 4.8 4.9 4.9 5.0 5.1
1.6 1.9 2.2 2.4 2.7 2.9 3.2 3.4 3.6 3.7 3.9 4.1 4.2 4.3 4.4 4.6 4.6 4.7 4.8 4.8
10.7 11.9 13.1 14.2 15.3 16.4 17.4 18.4 19.4 20.3 21.2 22.1 22.9 23.7 24.5 25.2 25.9 26.6 27.2 27.8
Estimated fetal weight = 271.45 – 53.886GA + 0.0029413GA2; therefore R2 = 0.887 HC/AC ratio = 1.2976 – 0.0068615GA; therefore R2 = 0.221 Table I demonstrates the expected value of BPD, HC, bicerebellar diameter, humerus length, FL, tibia length, fibula length, AC, HC/AC ratio, and estimated fetal weight predicted by the regression equations at each week of gestation from 16 to 35 weeks. When estimated fetal weight calculated by BPD and AC was compared with estimated fetal weight calculated from FL and AC, no significant differences were noted. As such, we chose to use the average of the 2 methods, because one cannot always obtain all measurements in high-order multifetal
pregnancies. The percent discordance for estimated fetal weight is also dependent on gestational age (R2 = 0.059). The 95th percentile of the confidence interval for percent discordance is 25% at 18 weeks and increases to 32% by 32 weeks. Comment It is well established that fetal growth is best assessed on ultrasonographic measurements generated from the population of the individuals under study. When such data are not available, curves from a population similar to the one under study should be used. Such curves are readily available for singleton and twin gestations.7, 8 However, a comprehensive set of these parameters has
1132 Rodis et al
not been available from a cohort of triplet fetuses from the United States. It is also clear that fetal growth and birth weight in triplets do not follow growth curves of either singletons or twins, as demonstrated recently by Alexander et al,9 using data from the National Center for Health Statistics. By comparing birth weights of singletons born alive in 1995 with birth weights of twins and triplets born alive from 1991 to 1995, they demonstrated that fetal growth in triplets begins to deviate from that of singletons and twins at approximately 31 weeks, after which the birth weight was significantly lower at each gestational age. Triplet pregnancies are at very high risk of preterm birth for numerous reasons, in particular preterm labor and preeclampsia, as our patients demonstrated. Because one would have no way of knowing which triplet pregnancy is destined to result in which of these complications, stratifying the data by indication for delivery or excluding such patients from analysis would bias the sample and make it clinically less useful. We chose to include all patients in our analysis, regardless of the indication for delivery, because they were ascertained prospectively. Large data sets of growth measurements in triplets, with more measurements at each week of gestation, obtained from the local population would be ideal. However, because such data sets may not be available, we believe that the graphs and table gener-
November 1999 Am J Obstet Gynecol
ated from this comprehensive set of fetal growth measurements can be used to assess longitudinal fetal growth of triplets. REFERENCES
1. Ventura SJ, Martin JA, Clarke SC, Mathews TJ. Report of final natality statistics, 1995. Hyattsville (MD): National Center for Health Statistics; 1997 Vol 31(Suppl 2). Monthly Vital Statistics Report No.: 11. 2. Martin JA, MacDorman MF, Mathews TJ. Triplet births: trends and outcomes, 1971-1994. National Center for Health Statistics. Vital Health Stat 21 1997:55. 3. Weissman A, Jakobi P, Yoffe N, Zimmer EZ, Paldi E, Brandes JM. Sonographic growth measurements in triplet pregnancies. Obstet Gynecol 1990;75:324-8. 4. Mordel N, Laufer N, Zajicek G, Shalev Z, Levin A, Schenker JG, et al. Sonographic growth curves of triplet conceptions. Am J Perinatol 1993;10:239-42. 5. Shepard MJ, Richards VA, Berkowitz RL, Warsof SL, Hobbins JC. An evaluation of two equations for predicting fetal weight by ultrasound. Am J Obstet Gynecol 1982;142:47-54. 6. Hadlock FP, Harrist RB, Carpenter RJ, Deter RL, Park SK. Sonographic estimation of fetal weight: the value of femur length in addition to head and abdomen measurements. Radiology 1984;150:535-40. 7. Hadlock FP, Deter RL, Harrist RB, Park SK. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology 1984;152:497-501. 8. Rodis JF, Vintzileos AM, Campbell WA, Pinette MG, Nochimson DJ. Intrauterine fetal growth in concordant twin gestations. Am J Obstet Gynecol 1990;162:1025-9. 9. Alexander GR, Kogan M, Martin J, Papiernik E. What are the fetal growth patterns of singletons, twins, and triplets in the United States? Clin Obstet Gynecol 1998;41:114-25.