Estimation of fetal weight in twins: a new mathematical model

Twin charts should be the standard reference to assess growth in twin pregnancy

PURPOSE OF REVIEW

Twin pregnancies are characterized by slower growth velocity compared with singletons, especially during the third trimester and, therefore, tend to be smaller than singletons. Growth surveillance is instrumental in the identification of twin pregnancies at risk of adverse outcomes. Whether the phenomenon of slow growth is an adaptive physiological response or represents pathological growth lag, is controversial. In this review, we focus on the evidence related to the types of growth charts employed for twin pregnancies.

RECENT FINDINGS

Consistent evidence has emerged over the past few years from large independent cohorts in different countries suggesting that twin-specific standards perform better in identifying growth-restricted twin pregnancies at risk of adverse perinatal outcomes, without resulting in an increase in interventions or iatrogenic prematurity.

SUMMARY

The current evidence supports the use of twin-specific reference charts. Concerted efforts should be made to derive prospective evidence from large multicentre studies on various aspects of the implementation of twin-specific standards.

The value of ultrasound in predicting isolated inter-twin discordance and adverse perinatal outcomes

PURPOSE

To investigate the value of ultrasound approaching delivery to predict isolated inter-twin discordance and adverse perinatal outcomes.

METHODS

We retrospectively included twin pregnancies with sonography approaching delivery in ten maternal-foetal medicine centres in China from 2013 to 2014. Estimated foetal weight (EFW) and inter-twin EFW disparity (EFWD) were calculated based on biometry parameters. Percentage errors between EFW and actual birthweight or between EFWD and actual inter-twin disparity were calculated. ROC curves and multiple logistic regression were applied to evaluate the ability of EFWD to predict inter-twin disparity ≥ 25%, stillbirth, asphyxia and admission to a neonatal intensive unit (NICU). Chorionicity-stratified analysis was further performed.

RESULTS

Two hundred sixty-six monochorionic and 760 dichorionic twin pregnancies were analysed. The percentage errors in foetal weight estimations were 7-13%, whereas percentage errors in the estimation of inter-twin disparity were nearly 100%. Among eight formulas, Hadlock1 performed best, with a detectable rate of 65% and a false positive rate of 5% when predicting inter-twin disparity ≥ 25%. EFWD ≥ 22% was strongly associated with stillbirth (OR = 4.17, 95% CI 1.40-12.40) and NICU admission (OR = 3.48, 95% CI 2.03-5.97) after adjustment for gestational age, parity and abnormal umbilical systolic/diastolic ratio. Ultrasound had better predictive ability in monochorionic twins.

CONCLUSION

The predictive value of ultrasound for isolated inter-twin discordance and adverse perinatal outcomes was limited, which was possibly due to the magnifying of systematic errors in the disparity estimation compared with weight estimation. Despite this, abnormal biometry was an independent contributor for the poor prognosis of neonates.

Comparison of conventional 2D ultrasound to magnetic resonance imaging for prenatal estimation of birthweight in twin pregnancy

BACKGROUND

During prenatal follow-up of twin pregnancies, accurate identification of birthweight and birthweight discordance is important to identify the high-risk group and plan perinatal care. Unfortunately, prenatal evaluation of birthweight discordance by 2-dimensional ultrasound has been far from optimal.

OBJECTIVE

The objective of the study was to prospectively compare estimates of fetal weight based on 2-dimensional ultrasound (ultrasound-estimated fetal weight) and magnetic resonance imaging (magnetic resonance-estimated fetal weight) with actual birthweight in women carrying twin pregnancies.

STUDY DESIGN

Written informed consent was obtained for this ethics committee-approved study. Between September 2011 and December 2015 and within 48 hours before delivery, ultrasound-estimated fetal weight and magnetic resonance-estimated fetal weight were conducted in 66 fetuses deriving from twin pregnancies at 34.3-39.0 weeks; gestation. Magnetic resonance-estimated fetal weight derived from manual measurement of fetal body volume. Comparison of magnetic resonance-estimated fetal weight and ultrasound-estimated fetal weight measurements vs birthweight was performed by calculating parameters as described by Bland and Altman. Receiver-operating characteristic curves were constructed for the prediction of small-for-gestational-age neonates using magnetic resonance-estimated fetal weight and ultrasound-estimated fetal weight. For twins 1 and 2 separately, the relative error or percentage error was calculated as follows: (birthweight - ultrasound-estimated fetal weight (or magnetic resonance-estimated fetal weight)/birthweight) × 100 (percentage). Furthermore, ultrasound-estimated fetal weight, magnetic resonance-estimated fetal weight, and birthweight discordance were calculated as 100 × (larger estimated fetal weight-smaller estimated fetal weight)/larger estimated fetal weight. The ultrasound-estimated fetal weight discordance and the birthweight discordance were correlated using linear regression analysis and Pearson's correlation coefficient. The same was done between the magnetic resonance-estimated fetal weight and birthweight discordance. To compare data, the χ², McNemar test, Student t test, and Wilcoxon signed rank test were used as appropriate. We used the Fisher r-to-z transformation to compare correlation coefficients.

RESULTS

The bias and the 95% limits of agreement of ultrasound-estimated fetal weight are 2.99 (-19.17% to 25.15%) and magnetic resonance-estimated fetal weight 0.63 (-9.41% to 10.67%). Limits of agreement were better between magnetic resonance-estimated fetal weight and actual birthweight as compared with the ultrasound-estimated fetal weight. Of the 66 newborns, 27 (40.9%) were of weight of the 10th centile or less and 21 (31.8%) of the fifth centile or less. The area under the receiver-operating characteristic curve for prediction of birthweight the 10th centile or less by prenatal ultrasound was 0.895 (P < .001; SE, 0.049), and by magnetic resonance imaging it was 0.946 (P < .001; SE, 0.024). Pairwise comparison of receiver-operating characteristic curves showed a significant difference between the areas under the receiver-operating characteristic curves (difference, 0.087, P = .049; SE, 0.044). The relative error for ultrasound-estimated fetal weight was 6.8% and by magnetic resonance-estimated fetal weight, 3.2% (P < .001). When using ultrasound-estimated fetal weight, 37.9% of fetuses (25 of 66) were estimated outside the range of ±10% of the actual birthweight, whereas this dropped to 6.1% (4 of 66) with magnetic resonance-estimated fetal weight (P < .001). The ultrasound-estimated fetal weight discordance and the birthweight discordance correlated significantly following the linear equation: ultrasound-estimated fetal weight discordance = 0.03 + 0.91 × birthweight (r = 0.75; P < .001); however, the correlation was better with magnetic resonance imaging: magnetic resonance-estimated fetal weight discordance = 0.02 + 0.81 × birthweight (r = 0.87; P < .001).

CONCLUSION

In twin pregnancies, magnetic resonance-estimated fetal weight performed immediately prior to delivery is more accurate and predicts small-for-gestational-age neonates significantly better than ultrasound-estimated fetal weight. Prediction of birthweight discordance is better with magnetic resonance imaging as compared with ultrasound.

Diagnostic accuracy of ultrasound in predicting birth-weight discordance in twin pregnancy: systematic review and meta-analysis

OBJECTIVE

To investigate the diagnostic performance of ultrasound in predicting birth-weight (BW) discordance in twin pregnancy.

METHODS

Electronic databases PubMed, EMBASE and CINAHL were searched using combinations of MeSH terms 'birth weight', 'discordance', 'twins' and 'ultrasound'. Summary estimates of sensitivity, specificity, positive and negative likelihood ratios and diagnostic odds ratios for the predictive accuracy of sonographically estimated fetal-weight (EFW) discordance and abdominal-circumference (AC) discordance in predicting BW discordance were computed using hierarchical summary receiver-operating characteristics curves.

RESULTS

Twenty studies (including 5826 twin pregnancies) were included. EFW discordance ≥ 20% had a sensitivity of 65.4% and specificity of 90.8% in predicting BW discordance ≥ 20%. The predictive performance of ultrasound performed within 1 month, 2 weeks and 3 days before birth was 61.4%, 72.3% and 78.9%, respectively. EFW discordance ≥ 25% had a sensitivity of 57.7% and specificity of 95.2% in predicting BW discordance ≥ 25%. The sensitivity of EFW discordance ≥ 25% detected within 1 month, 2 weeks and 3 days before birth was 60.0%, 75.0% and 60.3%, respectively, while the corresponding values for specificity were 97.7%, 96.2% and 87.3%, respectively. In view of the different cut-offs reported among studies, it was not possible to perform comprehensive data synthesis for each AC discordance cut-off. The optimal diagnostic performance of AC discordance was for prediction of BW discordance ≥ 25%, with a sensitivity of 70.8% and specificity of 86.4%.

CONCLUSION

Ultrasound EFW discordance has an overall moderate accuracy in predicting BW discordance in twin pregnancy. Copyright © 2016 ISUOG. Published by John Wiley & Sons Ltd.

A new mathematical formula to predict the foetal weight in twin pregnancies: A comparison of it with 19 different formulas

One hundred and seventy-two twin-pregnant patients were enrolled. The estimated foetal weight was calculated using 19 different formulas. Ong's formula (0.954 (95%CI = 0.938/0.966)), which was designed specifically for twins, produced the highest Cronbach's alpha value followed by Hadlock II (0.952 (95%CI = 0.935/0.965)), Hadlock I (0.952 (95%CI = 0.935/0.964)), Hadlock III (0.952 (95%CI = 0.935/0.964)), Hadlock IV (0.952 (95%CI = 0.935/0.964)) and our formula (0.952 (95%CI = 0.935/0.964)), which produced the same Cronbach's alpha values for twin A. For twin B, our formula produced the highest Cronbach's alpha value (0.961 (95%CI = 0.948/0.972) followed by Hadlock II (0.960 (95%CI = 0.946/0.971)), Hadlock I (0.960 (95%CI = 0.946/0.970)), Hadlock III (0.960 (95%CI = 0.946/0.970)) and Hadlock IV (0.960 (95%CI = 0.946/0.970)). In conclusion, our formula (AC, FL) performed well in predicting the foetal weights in twin pregnancies (>24 weeks) in our study. However, it should be tested in other populations. Hadlock II (AC, FL) produced a comparable performance to Hadlock I (BPD, HC, AC, FL), Hadlock III (BPD, AC, FL) and Hadlock IV (HC, AC, FL). Hadlock II may be preferable in twin pregnancies since it is based on AC and FL only.

Ultrasound estimation of birth weight in twin pregnancy: comparison of biometry algorithms in the STORK multiple pregnancy cohort

OBJECTIVES

The aims of this study were first, to ascertain the accuracy of formulae for ultrasonographic birth-weight estimation in twin compared with singleton pregnancies and second, to assess the accuracy of sonographic examination in the prediction of birth-weight discordance in twin pregnancies.

METHODS

This was a retrospective cohort study including both singleton and twin pregnancies. Routine biometry was recorded and estimated fetal weight (EFW) calculated using 33 different formulae. Only pregnancies that delivered within 48 h of the ultrasound scan were included (4280 singleton and 586 twin fetuses). Differences between the EFW and actual birth weight (ABW) were assessed by percentage error, accuracy in predictions within ± 10% and ± 15% of error and use of the Bland-Altman method. The accuracy of prediction of the different cut-offs of birth-weight discordance in twin pregnancies was also assessed using the area under the receiver-operating characteristics curve (AUC).

RESULTS

The overall mean absolute percentage error was ≤ 10% for 25 formulae in singleton pregnancies compared with three formulae in twin pregnancies. The overall predictions within ± 10% and ± 15% of the ABW were 62.2% and 81.5% in singleton and 49.7% and 68.5% in twin pregnancies, respectively. When t e formulae were categorized according to the biometric parameters included, those based on a combination of head, abdomen and femur measurements showed the lowest mean absolute percentage error, in both singleton and twin pregnancies. The predictive accuracy for 25% birth-weight discordance using the Hadlock 2 formula, as assessed by the AUC, was 0.87.

CONCLUSIONS

Ultrasound estimation of birth weight is less accurate in twin than in singleton pregnancies. Formulae that include a combination of head, abdomen and femur measurements perform best in both singleton and twin pregnancies.

A modified prenatal growth assessment score for the evaluation of fetal growth in the third trimester using single and composite biometric parameters

OBJECTIVE

To define modified Prenatal Growth Assessment Scores (mPGAS) for single and composite biometric parameters and determine their reference ranges in normal fetuses.

METHODS

Nine anatomical parameters (ap) were measured and the weight estimated (EWTa, EWTb) in a longitudinal study of 119 fetuses with normal neonatal growth outcomes. Expected third trimester size trajectories, obtained from second trimester Rossavik size models, were used in calculating Percent Deviations (% Dev's) and their age-specific reference ranges in each fetus. The components of individual % Dev's values outside their reference ranges, designated +iapPGAS, -iapPGAS, were averaged to give +apPGAS and -apPGAS values for the 3rd trimester. The +iapPGAS and -iapPGAS values for different combinations of ap (c1a (HC, AC, FDL, ThC, EWTa), c1b (HC, AC, FDL, ThC, EWTb), c2 (ThC, ArmC, AVol, TVol), c3 (HC, AC, FDL, EWTa)) were then averaged to give +icPGAS and -icPGAS values at different time points or at the end of the third trimester (+cPGAS, -cPGAS). Values for iapPGAS, ic1bPGAS, and ic2PGAS were compared to their respective apPGAS or cPGAS reference ranges.

RESULTS

All mPGAS values had one 95% range boundary at 0.0%. Upper boundaries of 1D +apPGAS values ranged from 0.0% (HC) to +0.49% (ThC) and were +0.06%, +2.3% and +1.8% for EWT, AVol and TVol, respectively. Comparable values for -apPGAS were 0.0% (BPD, FDL, HDL), to -0.58% (ArmC), -0.13% (EWT), -0.8% (AVol), and 0.0% (TVol). The +cPGAS, 95% reference range upper boundaries varied from +0.36% (c1b) to +0.89% (c2). Comparable values for -cPGAS lower boundaries were -0.17% (c1b) to -0.43% (c2).

CONCLUSIONS

The original PGAS concept has now been extended to individual biometric parameters and their combinations. With the standards provided, mPGAS values can now be tested to see if detection of different types of third trimester growth problems is improved.

Customized growth charts for twin gestations to optimize identification of small-for-gestational age fetuses at risk of intrauterine fetal death

OBJECTIVE

Customized growth charts developed for singleton pregnancies have been shown to be more effective than population-based ones at identifying small-for-gestational age (SGA) fetuses at risk for intrauterine fetal death (IUFD). We sought to compare the association between SGA and IUFD in twins using customized growth charts designed for twin gestations compared to those designed for singletons.

METHODS

This was a retrospective cohort study using a database including singleton and twin pregnancies undergoing ultrasound examination between 16 and 20 weeks' gestation. After excluding preterm births < 34 weeks, congenital anomalies and stillbirths, we identified 51, 150 singleton births. Coefficients for significant physiological and pathological variables affecting birth weight for singletons were derived using backward stepwise multiple regression. The same process was repeated for twin births (1608 pairs), also adjusting for chorionicity. Customized growth charts for each pregnancy were derived using these two regression models for optimal birth weight at term and a proportionality equation. The association between SGA < 10(th) percentile, defined using the twin and singleton-customized charts, and IUFD were compared. Statistical analysis, including calculation of adjusted odds ratios (OR) for IUFD and screening accuracy using each chart, was performed.

RESULTS

The derived coefficients for optimal birth weight for twins were different from those for singletons, with lower constants and root mean square error (3422 and 288.9, respectively, in twins vs 3543 and 416 in singletons). Among 3786 twin infants, IUFD was seen in 123 (3.2%). The numbers of fetuses identified as SGA were 575 (15.2%) and 504 (13.3%) by the singleton and twin charts, respectively. Fetuses classified as SGA by the twin-specific customized charts were at a significantly increased risk for IUFD (adjusted OR, 2.3 (95% CI, 1.4-3.5)), whereas those classified as SGA by the singleton-customized charts were not (adjusted OR, 1.2 (95% CI, 0.7-2.0)).

CONCLUSION

Customized charts designed specifically for twins are more effective at identifying twin pregnancies at risk for IUFD than are those derived using singleton birth data.

Sonographic accuracy of estimated fetal weight in twins

OBJECTIVES

We aimed to determine the accuracy of sonographic formulas for estimating birth weight in twin pregnancies.

METHODS

We conducted a retrospective cohort study of consecutive twin pregnancies undergoing sonography at within 1 week of delivery. Pregnancies were included if biometric measurements and birth weight were available and excluded if anomalies were present. Estimated fetal weight was calculated using three sonographic formulas: two derived from singletons (Hadlock and Shepard) and one from twins (Ong). The correlation between estimated fetal weight and birth weight was determined using the Pearson correlation coefficient. The accuracy of each formula (bias) was assessed using the mean percentage error [(estimated fetal weight - birth weight)/birth weight × 100], and the precision (random error) was estimated from the standard deviation of the percentage error. The screening efficiency of each formula for intrauterine growth restriction, defined as below the 10th percentile on the Alexander growth standard, was assessed. The effect of twin presentation was determined using a paired analysis.

RESULTS

Of 1744 consecutive twin pregnancies, 270 (540 infants) met inclusion criteria. The estimated fetal weight of all 3 formulas strongly correlated with the birth weight (Pearson r = 0.90 for Hadlock, 0.87 for Shepard, and 0.92 for Ong). Each formula had similar sensitivity (65%-70%) and specificity (85%-90%) for intrauterine growth restriction. For each formula, the correlation coefficient was similar between twins A and B (Pearson r = 0.85-0.93); however, the estimated fetal weight for twin A tended to underestimate birth weight, whereas the estimated fetal weight for twin B tended to overestimate birth weight.

CONCLUSIONS

Three widely used estimated fetal weight formulas, two derived from singletons and one from twins, perform equally well in estimating birth weight in twin gestations.

Validity of sonographic prediction of fetal weight and weight discordance in twin pregnancies

OBJECTIVES

The aims of this study were (1) to assess the accuracy of estimated fetal weight (EFW) in twins and (2) to assess the accuracy of sonographic examination to predict birth weight discordance (BWD).

METHODS

We retrospectively analyzed collected data on twin pregnancies between 2004 and 2007. All twin pregnancies with at least one ultrasound (US) examination within 15 days of delivery were included in this study. EFW was calculated according to Hadlock1, Hadlock2, Ong, Shepard and Warsof formulas. Mean and SD of the standardized errors and percentage of newborns with birth weight (BW) within 10% of EFW were calculated.

RESULTS

Two hundred eighty-three twin pregnancies were included. Mean and SD (%) of the standardized errors were 1.54 +/- 12.19, 0.19 +/- 11.87, 10.93 +/- 15.55, - 1.91 +/- 14.93 and 5.37 +/- 14.91 for Hadlock1, Hadlock2, Shepard, Ong and Warsof formulas, respectively. Hadlock2's formula allowed for the highest proportion of newborns with BW within 10% of EFW and it also performed best to predict discordance of more than 25% as assessed by area under the ROC curve.

CONCLUSIONS

Sonographic prediction of inter-twin BWD within 15 days of delivery seems to be accurate enough for routine clinical use. Performance and predictive values depend on the threshold chosen to define EFW and BW discordance.

New regression formulas for sonographic weight estimation within 10, 7, and 3 days of delivery

OBJECTIVES

The purpose of this study was to develop new regression formulas based on large numbers of sonographic examinations performed within 10, 7, and 3 days of delivery.

METHODS

Sonographic fetal biometric measurements and delivery ward data for an unselected population were analyzed. Multivariate linear regression models were fitted to the sonographic data to predict the actual birth weight (BW) within 10, 7, and 3 days.

RESULTS

The analyses included 6289, 5449, and 4007 patients who underwent sonographic examinations within 10, 7, and 3 days of delivery, respectively. All models yielded very high correlation coefficients (r = 0.927-0.958; R(2) = 0.859-0.918), low mean deviations between the calculated and actual BWs (6.4%-6.6% +/- 1 SD of 5.5%-5.9%), and high percentages of the calculated BW within 10% of the actual BW (78.5%-80.4%). Estimated fetal weight analyses made within 3 days of delivery yielded slightly better results than within 7 and 10 days.

CONCLUSIONS

The new regression formulas yielded overall similar results, with a small advantage for estimates calculated within 3 days of delivery. Further prospective studies are needed to compare the accuracy of these formulas with those used to date.

First and third trimester ultrasound in the prediction of birthweight discordance in dichorionic twins

OBJECTIVE

Birthweight discordance amongst twins is associated with an increase in perinatal morbidity and mortality. Clinical decisions regarding the management of twins are often made on the basis of estimated fetal weight in the third trimester. This study assesses the efficacy of routine third trimester ultrasound fetal biometry, third trimester fetal growth velocity and first trimester differences in size in the prediction of subsequent birthweight discordance.

STUDY DESIGN

Structurally and chromosomally normal dichorionic twin gestations resulting in two live births after 24 weeks gestation were identified. Ultrasound examinations of dichorionic twin pregnancies between 10 and 14 weeks of gestation were evaluated for inter-twin biometric disparity in crown-rump length (CRL). Estimated fetal weight (EFW) was calculated from the last scan before delivery. Fetal growth velocity (FGV) was calculated from two estimates of fetal weight between 21 and 35 days apart and incorporating the last EFW before delivery. EFW and FGV were expressed as standard deviation scores (Z scores). Disparity between these ultrasound parameters was correlated to inter-twin birthweight difference. Birthweight discordance was defined as > or =20% difference in birthweight relative to the heavier twin. Fishers exact and Spearman rho tests were used for statistical analysis. Test performance is expressed as area under the ROC curve and likelihood ratios (LR).

RESULTS

One hundred and thirty-five dichorionic gestations were identified over a consecutive 3-year-period. One hundred and eight twin gestations had both crown-rump length (CRL) and sufficient ultrasound measurements in the third trimester to calculate FGV. Twenty-six pregnancies (24%) had discordant birthweight. Only EFW Z score difference was correlated with birthweight disparity (r=0.36, p = <0.001). An EFW Z score difference of 0.6 had an area under the ROC curve of 0.70 (95% C.I. 0.59-0.79) but this did not differ significantly from first trimester prediction of growth discordance which had an area of 0.55 (95% C.I. 0.44-0.66). The LR for first and third trimester prediction of discordance were low.

CONCLUSION

Ultrasound measurement of fetal growth velocity and size both in the first and third trimesters is a poor predictor of birthweight discordance. Disparate fetal growth is exhibited as early as 10-14 weeks gestation and differences at this early stage in fetal development are no less accurate than EFW within 1 month of delivery. First trimester ultrasound, third trimester growth velocity and third trimester EFW do not accurately predict the delivery of twins with discordant birthweight.

A systematic review of the ultrasound estimation of fetal weight

OBJECTIVES

The range and use of ultrasound fetal measurements have gradually been extended. Measurements have been combined to estimate fetal weight by mathematically based non-linear regression analysis or physically based volumetric methods. Fetal weight estimation is inaccurate, with poor sensitivity for prediction of fetal compromise. Several authors have shown the unacceptable level of intra- and interobserver variability in fetal measurement and the impact of errors on growth assessment. The aims of this study were to review the available methods and possible sources of inaccuracy.

METHODS

Four databases were searched for studies comparing ultrasound estimated fetal weight (EFW) with birth weight. Studies meeting the inclusion criteria evaluated 11 different methods. Errors were graphically summarized.

RESULTS

No consistently superior method has emerged. Volumetric methods provide some theoretical advantages. Random errors are large and must be reduced if clinical errors are to be avoided.

CONCLUSIONS

The accuracy of EFW is compromised by large intra- and interobserver variability. Efforts must be made to minimize this variability if EFW is to be clinically useful. This may be achieved through averaging of multiple measurements, improvements in image quality, uniform calibration of equipment, careful design and refinement of measurement methods, acknowledgment that there is a long learning curve, and regular audit of measurement quality. Further work to improve the universal validity and accuracy of fetal weight estimation formulae is also required.

Fetal biometry: clinical, pathological, and technical considerations

Sonographic measurements of fetal ultrasound parameters are the basis for accurate determination of gestational age and detection of fetal growth abnormalities. Selection of the most useful single biometric parameter depends on the timing and purpose of measurement and is influenced by specific limitations. CRL (crown-rump length) is the best parameter for early dating of pregnancy. Biparietal diameter (BPD) maintains the closest correlation with gestational age in the second trimester. In cases of variation in the shape of the skull, head circumference is an effective alternative. Abdominal circumference is the most useful dimension to evaluate fetal growth, and femur length is the best parameter in the evaluation of skeletal dysplasia. Use of multiple predictors improves the accuracy of estimates. An individual approach to each pregnancy is recommended for fetal growth assessment. The various epidemiological factors involved in fetal growth should be considered and specific charts for different communities should be used when possible. The methods of fetal weight estimation with their limitations and potential errors are presented. Clinical application of fetal biometry in abnormal growth is discussed in cases of small- and large-for-gestational-age fetuses, chromosomal aberrations, and skeletal dysplasias.