Sonographic examination does not predict twin growth discordance accurately

Investigation of the Association of Abdominal Circumference Discord-ance and Estimated Fetal Weight Discordance in Twins with Birth Weight Discordance

Background

Twin pregnancy is associated with a high risk of mortality and morbidity. It is necessary to estimate the weight difference of the fetuses with a reliable method to prevent possible complications. This study was conducted to compare the association between the Estimated fetal weight (EFW) discord-ance and the Abdominal Circumference (AC) discordance with birth weight in twins.

Methods

This was a descriptive-analytical and retrospective study. The statistical population was all twin pregnant mothers referred to Imam Khomeini Hospital in Ahvaz from 2017 to 2019. The sample size was determined with a census (540 people). Based on AC , the size of head circumference (HC), femur length (FL), and the Biparietal Diameter (BPD), EFW was calculated. Then the EFW Dis-cordance and AC Discordance were calculated and compared with the birth weight. Data were analyzed using SPSS18. Unpaired, Two-Tailed T-test and Pearson correlation test were used.

Results

The results showed that the mean discordance of fetal weight in twin pregnancies in the EFW method was 9.25%, in the AC method was 9.89% and finally, at birth, was 10.72%. The correla-tion of the weight difference between the two embryos in the AC method with the time of birth (r = 0.922 and P < 0.001) was higher than in the EFW method with the time of birth (r = 0.69 and P < 0.001) and finally, it was found that in detecting the discordance more than 20% and 25%, AC diagnostic power was good, but EFW was moderate.

Conclusion

Therefore, to evaluate the weight and weight difference in twin embryos, the AC method has the appropriate accuracy and compatibility. Another major prospective study to evaluate the diagnostic performance of AC and EFW mismatch based on gestational age at scan, incision point, and maternal and placental characteristics to determine true ultrasound diagnostic accuracy in predict-ing growth mismatch in twin pregnancy and optimal post-case management option is needed.

Ultrasound for diagnosis of birth weight discordance in twin pregnancies

BACKGROUND

There is a need to standardize monitoring in obstetric research of twin pregnancies. Identification of birth weight discordance (BWD), defined as a difference in the birth weights of twins, is a well-documented phenomenon in twin pregnancies. Ultrasound for the diagnosis of BWD informs complex decision making including whether to intervene medically (via laser photo coagulation) or deliver the twins to avoid fetal morbidities or even death. The question is, how accurate is this measurement?

OBJECTIVES

To determine the diagnostic accuracy (sensitivity and specificity) of ultrasound estimated fetal weight discordance (EFWD) of 20% and 25% using different estimated biometric ultrasound measurements compared with the actual BWD as the reference standard in twin pregnancies.

SEARCH METHODS

The search for this review was performed on 15 March 2019. We searched CENTRAL, MEDLINE (Ovid), Embase (Ovid), seven other databases, conference proceedings, reference lists and contacted experts. There were no language or date restrictions applied to the electronic searches, and no methodological filters to maximize sensitivity.

SELECTION CRITERIA

We selected cohort-type studies with delayed verification that evaluated the accuracy of biometric measurements at ultrasound scanning of twin pregnancies that had been proposed for the diagnosis of estimated BWD, compared to BWD measurements after birth as a reference standard. In addition, we only selected studies that considered twin pregnancies and applied a reference standard for EFWD for the target condition of BWD.

DATA COLLECTION AND ANALYSIS

We screened all titles generated by electronic database searches. Two review authors independently assessed the abstracts of all potentially relevant studies. We assessed the identified full papers for eligibility, and extracted data to create 2 × 2 tables. Two review authors independently performed quality assessment using the QUADAS-2 tool. We excluded studies that did not report data in sufficient detail to construct 2 × 2 tables, and where this information was not available from the primary investigators. We assessed the certainty of the evidence using the GRADE approach.

MAIN RESULTS

We included 39 eligible studies with a median study sample size of 140. In terms of risk of bias, there were many unclear statements regarding patient selection, index test and use of proper reference standard. Twenty-one studies (53%) were of methodological concern due to flow and timing. In terms of applicability, most studies were of low concern. Ultrasound for diagnosis of BWD in twin pregnancies at 20% cut-off Twenty-two studies provided data for a BWD of 20% and the summary estimate of sensitivity was 0.51 (95% CI 0.42 to 0.60), and the summary estimate of specificity was 0.91 (95% CI 0.89 to 0.93) (8005 twin pregnancies; very low-certainty evidence). Ultrasound for diagnosis of BWD in twin pregnancies at 25% cut-off Eighteen studies provided data using a BWD discordance of 25%. The summary estimate of sensitivity was 0.46 (95% CI 0.26 to 0.66), and the summary estimate of specificity was 0.93 (95% CI 0.89 to 0.96) (6471 twin pregnancies; very low-certainty evidence). Subgroup analyses were possible for both BWD of 20% and 25%. The diagnostic accuracy did not differ substantially between estimation by abdominal circumference and femur length but femur length had a trend towards higher sensitivity and specificity. Subgroup analyses were not possible by sex of twins, chorionicity or gestational age due to insufficient data.

AUTHORS' CONCLUSIONS

Very low-certainty evidence suggests that EFWD identified by ultrasound has low sensitivity but good specificity in detecting BWD in twin pregnancies. There is uncertain diagnostic value of EFWD; this review suggests there is insufficient evidence to support this index as the sole measure for clinical decision making to evaluate the prognosis of twins with growth discordance. The diagnostic accuracy of other measures including amniotic fluid index and umbilical artery Doppler resistive indices in combination with ultrasound for clinical intervention requires evaluation. Future well-designed studies could also evaluate the impact of chorionicity, sex and gestational age in the diagnostic accuracy of ultrasound for EFWD.

Abdominal circumference discordance for prediction of small for gestational age at birth in twin pregnancies

OBJECTIVE

To evaluate whether single sonographic abdominal circumference (AC) discordancy estimation can predict small for gestational age (SGA) at birth in twin gestations.

METHODS

A retrospective analysis of prospectively collected data. Cohort included all twin gestations delivered at one university-affiliated medical center between 2010 and 2018, with available sonographic evaluation from 22 gestational weeks to term. Pregnancies complicated by fetal chromosomal abnormalities, major anomalies or twin to twin transfusion syndrome were excluded. One sonographic evaluation per pregnancy was selected randomly. AC discordance was calculated as (large twin AC - small twin AC)/large twin AC*100. Prediction of SGA at birth for at least one newborn (<10% percentile for gestational age by gender-specific local curves for multiples) was evaluated using ROC statistics with calculation of Youden index to establish best AC discordance cutoff. AC discordance prediction performance was compared to estimated fetal weight discordance performance. Results were adjusted for confounders using logistic regression analysis.

RESULTS

After exclusion, 236 twin gestations entered analysis. Of them, 200/236 (84.7%) were dichorionic-diamniotic twins. Mean gestational age at ultrasound evaluation and at delivery were 30.9 ± 4.4 and 35.9 ± 2.4 weeks, respectively. In 28/236 (11.8%) pregnancies, at least one neonate was born SGA. AC discordance predicted SGA at birth as good as sonographic estimated fetal weight (sEFW) discordance: ROC-AUC 0.76, 95% CI 0.67-0.85 vs. 0.77 95% CI 0.66-0.87, p < .001 for all. Best AC discordance cutoff for prediction of SGA at birth was 7.1% (57% sensitivity, 87% specificity), ROC-AUC 0.72 (95% CI 0.61-0.84, p < .001). Results remained significant after adjustment for maternal age, nulliparity, chorionicity and ultrasound to delivery interval (aOR 1.21 95% CI 1.1-1.32, p < .001).

CONCLUSION

According to our results, AC discordance at single sonographic evaluation can predict SGA at birth in twin gestations as good as sEFW discordance. Best cutoff for SGA prediction was 7.1%.

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.

Birth-weight discordance and neonatal morbidity in twin pregnancy: analysis of STORK multiple pregnancy cohort

OBJECTIVES

To investigate the relationship between weight discordance and neonatal morbidity in twin pregnancy progressing to at least 34 weeks of gestation. The secondary aim was to determine the predictive accuracy of different weight discordance cut-offs in predicting neonatal morbidity in twin pregnancy.

METHODS

This was a retrospective multicenter cohort study of all twin pregnancies booked for antenatal care at four hospitals in the Southwest Thames region of London Obstetric Research Collaborative (STORK) over a period of 10 years. Ultrasound data were obtained by a search of each hospital's obstetric ultrasound computer database, while outcome details were obtained from the computerized maternity and neonatal records. The primary outcome was incidence of composite neonatal morbidity in twin pregnancy with birth-weight discordance. Logistic regression was used to identify and adjust for potential confounders, while a receiver-operating characteristics (ROC) curve was used to determine predictive accuracy.

RESULTS

Nine hundred and thirty-nine twin pregnancies (760 dichorionic, 179 monochorionic) were included. Gestational age at birth and birth-weight decile were significantly lower in pregnancies complicated by neonatal morbidity compared with those which were not (P < 0.001 for both). On multivariable logistic regression analysis, gestational age at birth (P < 0.001), birth-weight decile (P = 0.029) and birth-weight discordance (P = 0.019), but not chorionicity (P = 0.477) or presence of at least one small-for-gestational-age (SGA) twin (P = 0.245), were associated independently with the risk of neonatal morbidity. There was a progressive increase in the risk of neonatal morbidity with increasing birth-weight discordance. Despite this association, birth-weight discordance showed an overall poor predictive accuracy for neonatal morbidity, with an area under the ROC curve of 0.58 (95% CI, 0.53-0.63) with an optimal cut-off of 17.6%, showing sensitivity and specificity of 35.2% (95% CI, 27.8-43.2%) and 83.2% (95% CI, 80.4-85.8%), respectively.

CONCLUSION

Intertwin birth-weight discordance is associated independently with the risk of neonatal morbidity in twins born after 34 weeks' gestation, irrespective of chorionicity or diagnosis of SGA in either twin. However, its predictive accuracy for neonatal morbidity is poor. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

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.

Accuracy of Prenatal Ultrasound in Detecting Growth Abnormalities in Triplets: A Retrospective Cohort Study

Significant management decisions in triplet pregnancies are made based mainly on ultrasound measurements of fetal growth, although there is a paucity of data examining the accuracy of fetal weight measurements in these gestations. To evaluate accuracy of prenatal ultrasound to diagnose growth abnormalities (intrauterine growth restriction, severe growth discordance) in triplet pregnancies, a retrospective cohort study of 78 triplet pregnancies (234 fetuses) delivered at a single tertiary hospital from January 2004 to May 2015 was performed. Growth percentiles from the last ultrasound were derived from estimated fetal weight using Hadlock's formula for each triplet. Growth discordance was calculated for each triplet set using the formula {(estimated fetal weight largest triplet - estimated fetal weight smallest)/estimated fetal weight largest}. These estimations were compared to birth weights. Sensitivity of ultrasound to predict ≥1 growth restricted fetus in a triplet set was 55.6% [95% CI 35.3, 74.5]; specificity was 100% [95% CI 93.0, 100]; positive predictive value (PPV) 100% [95% CI 74.7, 100]; negative predictive value (NPV) 81.0% [95% CI 73.2, 85.7%]. Sensitivity of ultrasound to detect fetal growth discordance >25% in a triplet set was 80.0% [95% CI 44.4, 97.5], specificity 94.1% [95% CI 85.6, 98.4]; PPV 66.7% [95% CI 42.4, 84.5]; NPV 97.0% [95% CI 90.2, 99.1]. Prenatal ultrasound currently remains the most reliable tool to screen for growth anomalies in triplet pregnancies; however, it appears to have less than ideal sensitivity, missing a number of cases of intra-uterine growth restriction and significant growth discordance.

Accuracy of Predicting Fetal Loss in Twin Pregnancies Using Gestational Age-Dependent Weight Discordance Cut-Offs: Analysis of the STORK Multiple Pregnancy Cohort

Objectives: A third-trimester fetal weight discordance of 25% has been proposed as an independent predictor of fetal loss in twin pregnancies. As fetal weight gain at this stage of pregnancy increases exponentially, it is not entirely certain whether a single cut-off for inter-twin weight discordance is appropriate. The aim of this study was to investigate whether a single weight discordance cut-off can be used or whether different cut-offs should be adopted according to the gestational age at assessment. Methods: This was a retrospective study of all twin pregnancies of known chorionicity from a large regional cohort over a 10-year period. Receiver operating characteristic curve and logistic regression analyses were used to explore the relation between estimated fetal weight (EFW) discordance detected within 4 weeks from the occurrence of the outcome and single fetal loss at different gestational age windows. Results: 957 twin pregnancies (173 monochorionic and 784 dichorionic) were included in the analysis. EFW discordance was independently associated with the occurrence of single fetal loss in twin pregnancies in each gestational age window. Ultrasound EFW discordance had an area under the curve of 0.77 (95% CI: 0.67-0.87) for the prediction of single fetal loss in the third trimester of pregnancy, with an optimal cut-off of around 25% (23.2%). The optimal cut-offs of EFW discordance for the prediction of single fetal loss were different in each gestational age window. Conclusion: The accuracy of EFW discordance in predicting single fetal loss in twin pregnancies varies during the third trimester of pregnancy. The degree of fetal weight discordance associated with fetal loss decreases during the third trimester, suggesting that the weight discordance threshold for intervention should vary according to gestational age.

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.

Weight discordance and perinatal mortality in twins: analysis of the Southwest Thames Obstetric Research Collaborative (STORK) multiple pregnancy cohort

OBJECTIVES

The degree of actual intertwin birth weight (BW) or ultrasound estimated fetal weight (EFW) discordance that justifies elective delivery is yet to be established. The main aim of this study was to ascertain the performance of BW and ultrasound EFW discordance in the prediction of perinatal loss in twin pregnancies.

METHODS

This was a retrospective study of all twin pregnancy births from a large regional cohort of nine hospitals over a 10-year period. Intertwin BW and ultrasound EFW discordance were analyzed in relation to the occurrence of stillbirth or neonatal death of one or both twins from 26 weeks' gestation as obtained from a mandatory national register. Receiver-operating characteristics (ROC), survival and logistic regression analyses were performed to evaluate the contribution of weight discordance in determining perinatal loss.

RESULTS

A total of 2161 twin pregnancies were included in the analysis. The area under the ROC curve for the prediction of perinatal loss was similar for BW and ultrasound EFW discordance (P = 0.62). Kaplan-Meier analysis showed that twins with BW or EFW of ≥ 25% discordance had a significantly lower survival trend than did those with lesser degrees of discordance (P < 0.001). The hazard ratios for the risk of total perinatal loss in twins with a BW or EFW discordance of ≥ 25% were 7.29 (95% CI, 4.37-12.00) and 7.28 (95% CI, 4.46-11.92), respectively. Logistic regression analysis demonstrated that BW discordance and gestational age, but not chorionicity or individual fetal size percentile, were independently associated with perinatal mortality.

CONCLUSIONS

An EFW discordance of ≥ 25% represents the optimal cut-off for the prediction of stillbirth and neonatal mortality irrespective of chorionicity or individual fetal size. A policy of increased fetal surveillance commencing from 26 weeks' gestation might be reasonable for pregnancies beyond this cut-off, but this would require confirmation in large-scale prospective trials.

Ultrasound in twin pregnancies

OBJECTIVE

To review the literature with respect to the use of diagnostic ultrasound in the management of twin pregnancies. To make recommendations for the best use of ultrasound in twin pregnancies.

OUTCOMES

Reduction in perinatal mortality and morbidity and short- and long-term neonatal morbidity in twin pregnancies. Optimization of ultrasound use in twin pregnancies.

EVIDENCE

Published literature was retrieved through searches of PubMed and the Cochrane Library in 2008 and 2009 using appropriate controlled vocabulary (e.g., twin, ultrasound, cervix, prematurity) and key words (e.g., acardiac, twin, reversed arterial perfusion, twin-to-twin transfusion syndrome, amniotic fluid). Results were restricted to systematic reviews, randomized control trials/controlled clinical trials, and observational studies. There were no date restrictions. Studies were restricted to those with available English or French abstracts or text. Searches were updated on a regular basis and incorporated into the guideline to September 2009. Grey (unpublished) literature was identified through searching the websites of health technology assessment and health technology assessment-related agencies, clinical practice guideline collections, clinical trial registries, and national and international medical specialty societies.

VALUES

The evidence collected was reviewed by the Diagnostic Imaging Committee of the Society of Obstetricians and Gynaecologists of Canada, with input from members of the Maternal Fetal Medicine Committee and the Genetics Committee of the SOGC. The recommendations were made according to the guidelines developed by The Canadian Task Force on Preventive Health Care (Table 1).

BENEFITS, HARMS, AND COSTS

The benefit expected from this guideline is facilitation and optimization of the use of ultrasound in twin pregnancy. SUMMARY STATEMENTS: 1. There are insufficient data to make recommendations on repeat anatomical assessments in twin pregnancies. Therefore, a complete anatomical survey at each scan may not be needed following a complete and normal assessment. (III) 2. There are insufficient data to recommend a routine preterm labour surveillance protocol in terms of frequency, timing, and optimal cervical length thresholds. (II-2) 3. Singleton growth curves currently provide the best predictors of adverse outcome in twins and may be used for evaluating growth abnormalities. (III) 4. It is suggested that growth discordance be defined using either a difference (20 mm) in absolute measurement in abdominal circumference or a difference of 20% in ultrasound-derived estimated fetal weight. (II-2) 5. Although there is insufficient evidence to recommend a specific schedule for ultrasound assessment of twin gestation, most experts recommend serial ultrasound assessment every 2 to 3 weeks, starting at 16 weeks of gestation for monochorionic pregnancies and every 3 to 4 weeks, starting from the anatomy scan (18 to 22 weeks) for dichorionic pregnancies. (II-1) 6. Umbilical artery Doppler may be useful in the surveillance of twin gestations when there are complications involving the placental circulation or fetal hemodynamic physiology. (II-2) 7. Although many methods of evaluating the level of amniotic fluid in twins (deepest vertical pocket, single pocket, amniotic fluid index) have been described, there is not enough evidence to suggest that one method is more predictive than the others of adverse pregnancy outcome. (II-3) 8. Referral to an appropriate high-risk pregnancy centre is indicated when complications unique to twins are suspected on ultrasound. (II-2) These complications include: 1. Twin-to-twin transfusion syndrome 2. Monoamniotic twins gestation 3. Conjoined twins 4. Twin reversed arterial perfusion sequence 5. Single fetal death in the second or third trimester 6. Growth discordance in monochorionic twins. Recommendations 1. All patients who are suspected to have a twin pregnancy on first trimester physical examination or who are at risk (e.g., pregnancies resulting from assisted reproductive technologies) should have first trimester ultrasound performed. (II-2A) 2. Every attempt should be made to determine and report amnionicity and chorionicity when a twin pregnancy is identified. (II-2A) 3. Although the accuracy in confirmation of gestational age at the first and second trimester is comparable, dating should be done with first trimester ultrasound. (II-2A) 4. Beyond the first trimester, it is suggested that a combination of parameters rather than a single parameter should be used to confirm gestational age. (II-2C) 5. When twin pregnancy is the result of in vitro fertilization, accurate determination of gestational age should be made from the date of embryo transfer. (II-1A) 6. There is insufficient evidence to make a recommendation of which fetus (when discordant for size) to use to date a twin pregnancy. However, to avoid missing a situation of early intrauterine growth restriction in one twin, most experts agree that the clinician may consider dating pregnancy using the larger fetus. (III-C) 7. In twin pregnancies, aneuploidy screening using nuchal transluscency measurements should be offered. (II-2B) 8. Detailed ultrasound examination to screen for fetal anomalies should be offered, preferably between 18 and 22 weeks' gestation, in all twin pregnancies. (II-2B) 9. When ultrasound is used to screen for preterm birth in a twin gestation, endovaginal ultrasound measurement of the cervical length should be performed. (II-2A) 10. Increased fetal surveillance should be considered when there is either growth restriction diagnosed in one twin or significant growth discordance. (II-2A) 11. Umbilical artery Doppler should not be routinely offered in uncomplicated twin pregnancies. (I-E) 12. For defining oligohydramnios and polyhydramnios, the ultrasonographer should use the deepest vertical pocket in either sac: oligohydramnios when < 2 cm and polyhydramnios when > 8 cm. (II-2B).

Fetal growth disorders in twin gestations

Twin growth is frequently mismatched. This review serves to explore the pathophysiologic mechanisms that underlie growth aberrations in twin gestations, the prenatal recognition of abnormal twin growth, and the critical importance of stratifying management of abnormal twin growth by chorionicity. Although poor in utero growth of both twins may reflect maternal factors resulting in global uteroplacental dysfunction, discordant twin growth may be attributed to differences in genetic potential between co-twins, placental dysfunction confined to one placenta only, or one placental territory within a shared placenta. In addition, twin-twin transfusion syndrome represents a distinct entity of which discordant growth is a common feature. Discordant growth is recognized as an independent risk factor for adverse perinatal outcome. Intertwin birth weight disparity of 18% or more should be considered to represent a discordance threshold, which serves as an independent risk factor for adverse perinatal outcome. At this cutoff, perinatal morbidity is found to increase both for the larger and the smaller twin within a discordant pair. There remains uncertainty surrounding the sonographic parameters that are most predictive of discordance. Although heightening of fetal surveillance in the face of discordant twin growth follows the principles applied to singleton gestations complicated by fetal growth restriction, the timing of intervention is largely influenced by chorionicity.

Discordant twins: diagnosis, evaluation and management

Approximately 16% of twin gestations have discordance of at least 20%. We identified 14 risk factors for divergent growth that can be categorized as maternal, fetal, or placental. Determination of chorionicity and serial ultrasound evaluation with a high index of suspicion for divergent growth is required for the diagnosis and stratification of risk. The highest reported likelihood ratio for detection of discordance was 5.9 during the first trimester examination and 6.0 for the second trimester. Although our ability to identify discordant twins is limited, once suspected and at viable gestational age, these pregnancies should have antepartum testing. Discordant growth alone is not an indication for preterm birth. Although there are multiple publications on the increased morbidity and mortality rates with discordant growth, there is a paucity of reports on how to manage them optimally and deliver them in a timely manner.

Prediction of birth weight discordance in twin pregnancies by second- and third- trimester ultrasound

INTRODUCTION

Our purpose was to examine whether the prenatal prediction of a critical birth weight discordance (BWD) in twin pregnancies can be improved by using either different formulae for prenatal fetal weight estimation or single biometric measurements or by assessing the intertwin discordance in the second trimester rather than shortly before birth.

MATERIAL AND METHODS

We conducted a retrospective study involving 196 twin pregnancies with an ultrasound examination between 18 and 25 weeks of gestation and one within 14 days prior to delivery. The accuracy was assessed by comparing the prenatal intertwin discordance (PID) with the BWD. PID was estimated by 5 common fetal weight estimation formulae and by single biometric measurements prior to delivery and in the second trimester.

RESULTS

The fetal weight estimation accuracy was similar in mono- and dichorionic pregnancies and the smaller and the larger twin. PID was most accurate with the fetal weight estimation formulae prior to delivery. The second-trimester measurements resulted in an underestimation of the BWD. Detection and false-positive rates for a BWD ≥20% were about 65 and 15%.

DISCUSSION

About two thirds of the twin pregnancies with a relevant BWD can be detected prior to delivery. An optimal detection rate requires fetal weight estimation close to delivery.

Factors affecting the accuracy of ultrasonographical fetal weight estimation in twin pregnancies

OBJECTIVE

To determine the factors affecting the accuracy of ultrasonographic weight estimation in twins.

METHODS

152 sets of twins delivered vaginally, were included. Effects of fetal weights, inter-twin weight discordance, chorionicity, early rupture of membranes, intrauterine growth restriction, and presentations of twins on errors of estimated fetal weights were evaluated. The primary measures of estimated fetal weight accuracy compared were mean-percentage-error and the standart deviation (SD) of percentage errors.

RESULTS

Mean percentage errors for the first fetus (8.13 ± 6.82) and the second fetus (8.07 ± 6.88) were similar (p = 0.64). Random errors of both fetuses were also similar (p = 0.78). If one of the fetuses had IUGR, the percentage error and also the random error of that fetus would increase significantly. Different presentations and fetal gender combinations were similar for both types of errors of fetal weight estimation. A weak negative lineer relationship was found between the weight of the first fetus and its percentage error (r = -0.27, p = 0.04). A similar relation was present between the weight and percentage error of the second fetus (r = -0.29, p = 0.03). Percentage errors and also random errors of both fetuses were significantly higher if severe discordance was present between twins (p = 0.01 and p = 0.02, respectively).

CONCLUSIONS

IUGR, fetal weights, and inter-twin discordence are the factors affecting the accuracy of weight estimation by ultrasonography.

Abdominal circumference ratio for the diagnosis of intertwin birth weight discordance

OBJECTIVES

We assessed the accuracy of predicting severe twin birth weight discordance (>25%) using the estimated fetal weights (EFW) and abdominal circumference (AC) ratio.

METHOD

A cohort of twin gestations underwent ultrasound examinations within two weeks from birth. We focused on the accuracy of EFW and on the diagnosis of severe birth weight discordance by the difference in EFWs and the AC ratio.

RESULTS

The 661 eligible twin pairs included 51 (7.7%) severely discordant pairs. The accuracy of an EFW to predict the actual birth weight was quite poor, with an acceptable specificity (96.4%), but low sensitivity (28.6-40.5%), to detect severely discordant pairs, whereas an AC ratio of 1.3 detected these discordant pairs with sensitivity and specificity of 97.3-100% and 99.6-99.7%, respectively.

CONCLUSION

By comparing EFWs, 59.5-71.4% of discordant pairs >25% are missed, whereas an AC ratio >1.3 would identify almost all cases.

The source of error in the estimation of intertwin birth weight discordance

OBJECTIVE

to assess the accuracy of paired estimated fetal weights (EFWs) to predict three levels of twin birth weight discordance (>15%, >20% and >25%).

METHOD

a cohort of twin pairs underwent ultrasound examinations within 2 weeks from birth. We calculated the frequency of under- and overestimation of the actual birth weight (< or >10%, respectively) in the entire cohort as well in the subset of mono- and dichorionic pairs.

RESULTS

discordance was largely underestimated (observed 10.4 ± 0.8% compared to actual 19.2 ± 1.1%, P=0.001) because the larger twin was more frequently underestimated [30.6 vs. 17.7%, odds ratio (OR) 2.0, 95% confidence interval (CI) 1.1, 3.9 in the entire cohort, and 34.2 vs. 13.1%, OR 3.4, 95% CI 1.4, 8.4 in the dichorionic pairs]. Overall, the specificity for detecting the three levels of discordance was adequate (91.5-94.2%) but the sensitivity was poor (11.1-17.8%) and tended to decrease with increasing discordance level.

CONCLUSION

the poor ability of paired EFWs to diagnose birth weight discordance results from underestimation of the larger twin.

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.

Effect of increased body mass index on the accuracy of estimated fetal weight by sonography in twins

OBJECTIVE

The purpose of this study was to determine whether increasing body mass index (BMI) decreases the accuracy of sonographic estimations of fetal weight in twin gestations.

METHODS

A chart review was conducted, in which 361 charts of patients with twin gestations over a 2-year period were reviewed. A total of 194 patients had sonographic examinations for fetal weight within 6 days of delivery and were included in the analysis. The difference between the sonographically estimated fetal weight was compared with the actual birth weight for each twin and stratified for the patient's BMI.

RESULTS

There was a significant increasing trend in mean absolute percent errors with increasing BMIs in both twins (P< .05). The mean absolute percent errors for twin A were 6% for patients with a BMI of less than 25 and 9% for those with a BMI of greater than 30. The mean absolute percent errors for twin B were 6.7% for patients with a BMI of less than 25 and almost 11.7% for those with a BMI of greater than 30. There was a significantly increasing trend in mean absolute differences in grams for both twins with increasing gestational age, with almost a 4-fold increase from less than 28 weeks to greater than 36 weeks in both twins (P< .05).

CONCLUSIONS

Increasing maternal obesity decreases the accuracy of sonographically determined fetal weight in twin gestations, particularly for twin B.

Growth abnormalities and multiple gestations

Growth abnormalities which include intrauterine growth restriction and weight discordance between twins are common in pregnancies complicated by multiple gestations and may be associated with poor perinatal outcomes. Knowledge of chorionicity is paramount when managing a multiple pregnancy. Monochorionic twins are at greater risk than dichorionic twins for growth issues, which may result in long-term complications including adverse neurological sequelae for the offspring. The purpose of the following article is to define normal and abnormal growth in multiples. In addition, the management of growth abnormalities in relationship to chorionicity will be discussed.

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.

Prenatal sonographic prediction of twin growth discordance

Our aim was to evaluate the ability of prenatal ultrasound scans to predict fetal growth discordance, and to examine the correlation between fetal weight estimated by ultrasound with actual birthweight in twin pregnancies. The study consisted of 221 twin pregnancies with ultrasound fetal weight estimates based on Hadlock's 4 parameter formula. Prediction of intertwin birthweight discordance was examined at 4 different intervals between ultrasound examination and delivery (0–7 days, n = 96; 8–14 days, n = 66; 15–21 days, n = 58; 22–28 days, n = 59 pregnancies), with a total of 279 ultrasound examinations. Birthweight discordance was considered as a difference of 20% or greater. The correlation between fetal weight estimated between 0 and 7 days and actual birthweight was calculated by intraclass correlation coefficient. The predictive values for intertwin discordance of 20% or more in the 0 to 7 days group were: sensitivity = 93.6%, specificity = 79.4%, positive predictive value = 89.2%, negative predictive value = 87.1% and accuracy = 88.6%. In the groups with scans carried out between 8 and 14 days, 15 and 21 days, and 22 and 28 days, the sensitivity and accuracy values were 95.8% and 84.9%, 95.6% and 84.5%, 90.9% and 84.8%, respectively. Fetal growth discordance in twins can be accurately predicted by ultrasound examination performed up to 28 days before birth. There is a good correlation between fetal weight estimated between 0 and 7 days and actual birthweight.

Sonographic prediction of significant intertwin birth weight discordance

OBJECT

To evaluate the usefulness of sonographic prediction of significant birth weight discordance in twin pregnancies.

STUDY DESIGN

Included in the study were live twin pairs with a gestational age of 24 weeks at birth who had received a sonographic examination within 28 days of delivery. Correlation tests and ROC curves were used to evaluate the predictability of significant intertwin growth discordance.

RESULTS

A total of 575 twin pregnancies were included in the study. The estimated discordance by sonographic examination proved to be an effective predictor of discordant twin growth. The birth weight discordance estimated by sonography tended to underestimate the degree of discordant twins.

CONCLUSION

If a sonographic examination is done within 28 days of delivery, it is a reliable predictor of twin birth weight discordance at the following levels: 15%, 20%, 25% or 30%. In order to obtain a higher sensitivity, the cut-off value of estimated birth weight discordance needs to be decreased by 5%.

Prediction of intra-twin birth weight discordance by binary logistic regression analysis

AIMS

Identification of women at high risk of intra-twin birth weight discordance is helpful in obstetric care of these pregnancies. The aim of this study is to establish an intra-twin birth weight discordance prediction model.

METHODS

We created an intra-twin birth weight discordance prediction model by logistic regression, based on the 1995-1997 register twin birth data of the USA. The twin sets were randomly divided into two groups: group 1 to establish the prediction model and group 2 to validate the prediction model. Intra-twin birth weight discordance was defined as birth weight discordance > 25%. The prediction model was validated by receiver operating characteristic curve.

RESULTS

A birth weight discordance prediction model including maternal age (beta = 0.069), parity (beta = 0.250), fetal gender concordance (beta = 0.041), maternal hypertension (beta = 0.368), eclampsia (beta = 0.316), other medical complication (beta = 0.165), and smoking (beta = 0.164) was established, yielded a 0.558 area under the receiver operating characteristic curve. The sensitivity, specificity, and positive predictive values were 38.1, 69.7, and 10.8%, respectively, at the cut-off value of 0.09 in group 2.

CONCLUSION

A birth weight discordance prediction model that includes seven variables available during pregnancy has been established with acceptable diagnostic performance.

Growth Aberration in Multiple Pregnancy

Growth of twins and higher-order multiples is an exceptional metabolic challenge for the expecting mother. She is doing much more than a mother of a singleton in terms of nurturing, however. Metabolic requirements need adequate dietary intervention in the form of increased weight gain during early pregnancy. It is normal for multiples to be smaller than singletons. Being smaller than singletons does not necessarily mean that multiples are pathologically growth restricted. It is important to remember that twins and triplets have different growth patterns, and their growth should not be considered by using singleton standards. When a small-for-gestational-age fetus is suspected in a multiple pregnancy, it is advisable to follow or to treat the pregnancy as if it was an SGA singleton.

Discordant growth in twins

Discordant growth in twins contributes significantly to rates of perinatal morbidity and mortality. These rates vary according to chorionicity, timing of onset and severity. We have reviewed English language literature in Medline since 1980. It is clear that diagnosis of discordant growth has improved due to the use of serial ultrasound examination. Following the detection of differences in fetal size, diagnosis is facilitated by umbilical artery and fetal Doppler studies. Management options vary according to chorionicity, timing of onset and umbilical-fetal Doppler studies. The mode of delivery in discordant twins remains controversial. We conclude that ultrasound surveillance of twin gestations, combining serial biometry and selective Doppler studies, is effective in the recognition of significant intrauterine growth restriction in co-twins. Differences in etiology and management underscore the importance of establishing chorionicity routinely as soon as twin gestation is diagnosed.

Prediction of growth discordance in twins with the use of abdominal circumference ratios

OBJECTIVE

The purpose of this study was to assess the accuracy of sonographic prediction of twin birth weight discordance using the abdominal circumference ratio.

STUDY DESIGN

This was a prospective cohort of diamniotic twin gestations that underwent serial ultrasound examinations every 2 to 4 weeks from 11 to 38 weeks of gestation. Birth weight discordance was defined as > or =25% difference in birth weight, relative to the larger twin. The sensitivity, specificity, and predictive values for the abdominal circumference ratio were assessed for the prediction of growth discordance.

RESULTS

Of 503 diamniotic twin pregnancies, 64 pregnancies (12.7%) had discordant fetal growth. The abdominal circumference ratio could be measured consistently throughout gestation in 100% of twin pairs. Receiver operating curve analysis showed that the abdominal circumference ratio was a good predictor of birth weight discordance (area under the curve = 0.80). An abdominal circumference ratio cutoff of 0.93 yielded a sensitivity and specificity of 61% and 84%, respectively.

CONCLUSION

Twin birth weight discordance may be predicted at any gestational age with an abdominal circumference ratio <0.93.

Is it normal for multiples to be smaller than singletons?

This section discusses fetal growth in multiple pregnancy from various perspectives. Whereas the entire 'fetal mass' of a multiple pregnancy can exceed the 90th birth weight percentile of a singleton of the same gestational age, the individual fetuses might exhibit growth patterns compatible with adaptation to the limited uterine environment. Adaptation can take the form of relative growth restriction (i.e. discordance), whereby not all fetuses show a decelerating growth pattern. When adaptation fails, fetal growth is genuinely restricted. However, even in cases with significant discordance, about 40% of the smaller twins weigh above the 10th birth weight percentile. The key in these circumstances might be found in the ability to recognize the transition from the physiologic adaptation into the pathologic process of growth restriction.

Neonatal outcome of growth discordant twin gestations

The aim of the study was to compare a variety of neonatal outcome variables of growth concordant twin gestations (CT) to that of growth discordant twins (DT). Maternal and neonatal charts of live, non-anomalous twins > 25 weeks' gestation from 1984-2000 with no evidence of twin-twin transfusion syndrome were reviewed for several variables. DT occurred in (N = 81) 11.9% of all twin pregnancies. In 61.7% of DT, twin B was the smaller of the twins. There was no difference in maternal age, admission indications, or antepartum complications between both groups. DT had a significantly higher incidence of growth restriction compared to CT (88.9% vs 43.5%, p < 0.001). More mothers of DT required oxytocin (37.0% vs 26.3%, p = 0.024); however, cesarean delivery rate and indications were similar in both groups. A similar percentage of infants had AS < 4 at 1 min and AS < 7 at 5 min in both groups. There was no difference between the 2 groups in neonatal complications including: trauma, respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis, pneumonia, seizures, or neonatal mortality. However, DT had a significantly higher incidence of hyperbilirubinemia, need for mechanical ventilation and a longer nursery stay. The neonatal outcome of growth discordant twins is worse than that of concordant twins even in pregnancies uncomplicated by twin-twin transfusion syndrome or congenital anomalies.

Positive predictive value of suspected growth aberration in twin gestations

OBJECTIVE

Our purpose was to determine the positive predictive value of ultrasonographic surveillance for growth abnormalities in twin gestations as a function of gestational age.

STUDY DESIGN

Women with twin gestations and delivery between January 1992 and March 1998 who had a 20- to 24-week sonogram with normal fetal anatomic findings and who had at least 1 sonogram showing abnormal growth were identified. Abnormal growth on ultrasonography was defined as an estimated fetal weight <10th percentile, abdominal circumference <5th percentile, or twin discordance (>20% difference in twin weights as a function of the heavier twin). Birth weights were then assessed for evidence of twin discordance or growth restriction.

RESULTS

The positive predictive value for the occurrence of a growth abnormality at birth, after an abnormal growth finding on ultrasonography at any time during gestation, was 47.7%. The positive predictive value was greatest (85%) when suspected growth restriction was first documented at 20 to 24 weeks of gestation and decreased with increasing gestational age. Even though sonograms were obtained at a mean interval of 4.4 +/- 2.0 weeks, those gestations with normal growth at 20 to 24 weeks had an elapsed time of 10.3 +/- 3.9 weeks until a growth abnormality was subsequently detected.

CONCLUSION

In twin gestations the positive predictive value of a sonogram for a growth abnormality at birth is significantly decreased after normal findings on a 20- to 24-week sonogram. This finding suggests that a routine 2- to 4-week interval between sonograms for all twin gestations may be unwarranted.

Estimation of fetal weight in twins: a new mathematical model

OBJECTIVES

Evaluation of new mathematical formula (Femur 4) derived from a twin population to estimate fetal weight in twins using ultrasound. Comparison of Femur 4 is with conventional mathematical models.

DESIGN

Retrospective analysis of ultrasonic measurements of 297 twin babies from 24 to 40 weeks of gestation who were born within 10 days of ultrasound examination.

SETTING

Aberdeen Maternity Hospital.

METHODS

With ultrasonic measurements obtained from twin babies, estimated fetal weight was calculated using the mathematical models of Campbell, Shepard and Hadlock. The calculations were repeated for the model of Femur 4. All models were compared against Femur 4.

RESULTS

The coefficient of determination of the linear regression between the actual and predicted weight was highest for Femur 4 (0.852). Femur 4 had the highest proportion of babies with estimated weights within 10% of actual birthweight (71.4%). In babies who weighed between 2000 and 3000 g, Femur 4 had the least systematic and random error of -1.69 and 8.96, respectively. For babies below the 10th centile for weight, Femur 4 had comparable positive and negative predictive values of 76.0% and 92.3%, respectively. Femur 4 was equally poor at predicting growth discordancy with positive and negative predictive values of 70.0% and 86.5% only.

CONCLUSION

Femur 4 requires measurements of femur length and abdominal circumference only, hence avoiding the need to obtain difficult head measurements which is a common problem in twins. It is a good model for estimation of fetal weight in twins. However, prediction of growth discordancy remains problematic.

Twin differences and similarities of birthweight and term in the French Romulus population

This study was performed to examine the main characteristics of the French Romulus twin population: zygosity, chorionicity, sex, term and birthweight. A sample of 104 pairs of twins was distinguished by zygosity, chorionicity and sex, and divided into concordant and discordant birthweight groups. Fifty-three % (n = 55) of the fetus pairs studied were born at "ideal term" (35-38 weeks), and 25% (n = 26) were delivered at "preterm" (28-34 weeks). The very preterm (< 28 weeks), and "postterm" (> 38 weeks) represented altogether 22% (n = 23) of the sample. Of the 104 twin pairs, 68% (n = 71) differ by less than 15% in birthweight, and 32% (n = 33) have a birthweight difference higher than 15%. In dizygotic (DZ) pairs females had more tendency to be in the discordant group (p = 0.01) while in monochorionic-monozygotic (MC-MZ) pairs males were more discordant (p = 0.07). We found a significant interaction between sex and zygosity type (p = 0.02). Males had a birthweight difference significantly weaker than that of females in dichorionic-monozygotic (DC-MZ) and DZ twins whereas it was higher than that of females in MC-MZ twins. There were no MZ twin pairs with DC placentation over than 15% birthweight difference. Log linear analysis demonstrated a three-way interaction (p < 0.05) between term type, zygosity type and hypotrophy. Our data indicate that in the group of twins born between 35 and 38 weeks' gestation the crucial question still remains unsettled on how term and birthweight are related to zygotism and/or chorionicity.

Sonographic considerations with multiple gestation

Determination of chorionicity is of paramount importance in risk assessment and management. Best performed in the first trimester, dichorionic placentation can be reliably assumed when the membrane is easily seen, there is a "twin peak" sign, there are clearly separate placentas, and there is discordant fetal gender. In a monochorionic twin pregnancy, there is a single placental mass, the dividing membrane is difficult to visualize until the end of the first trimester, and the membrane inserts onto the placental surface without a peaked appearance. Amniotic fluid volume assessment is important in the management of twin pregnancy. Polyhydramnios-oligohydramnios may be a manifestation of twin-twin transfusion syndrome, although oligohydramnios with normal amniotic fluid volume in the other twin's sac may more likely be a sign of velamentous cord insertion, infection, or chromosomal or structural abnormality. Fetal growth discordance is common in twin pregnancy and is associated with increased perinatal mortality and morbidity. The most sensitive indicator of discordant twin growth is thought to be estimated fetal weight, and an intertwin difference of > or = 20% is considered significant. In the clinical care of a patient with twins, it is reasonably standard to confirm chorionicity with ultrasonography in the first or early second trimester. At about 20 weeks, a level II ultrasound for anatomic survey is indicated. In dichorionic pregnancies, ultrasound examinations are then performed at 26 to 28 weeks and every 3 to 4 weeks thereafter to follow growth and amniotic fluid volume. In monochorionic twins, we generally do an additional ultrasound at about 23 to 24 weeks, because of the risk of twin-twin transfusion syndrome. In the late third trimester, careful attention should also be given to fetal position, to help with delivery planning.

Ambulatory management of multiple gestation

Ambulatory management of multiple gestation requires careful and continuing care by the obstetrician. The initial evaluation should include a comprehensive history, including use of fertility enhancing drugs and ART, family history, social history; a general physical examination, including a pelvic examination; laboratory evaluation, including complete blood cell count, dipstick urinalysis for protein and glucose, urine culture, blood type, Rh factor and irregular blood antibody determination, serology for rubella, syphilis, hepatitis B surface antigen and varicella (if there is no history). A Papanicolaou smear should be done at the time of the pelvic examination, as should evaluation for bacterial vaginosis. Ultrasound assessment of placentation should be done at 14 weeks' gestation, but vaginal or perineal ultrasound of cervical length should be done at the initial visit. Other testing procedures should include repeat ultrasound evaluation for fetal growth every 4 weeks in a dichorionic placentation and every 3 weeks if monochorionic placentation is present. Triple screen MSAFP at 16-18 weeks' gestation and blood sugar screening at 22-26 weeks should be performed. After the first trimester, the patient should schedule physician visits every 2 weeks or less. Routine medications should include one prenatal vitamin per day, additional folic acid supplementation of 1.0 mg per fetus, supplemental iron preparation, and additional calcium to equal 1500 mg/day. The use of low-dose aspirin to prevent preeclampsia in twin gestations has not been adequately studied. Continuing vigilance by the knowledgeable obstetrician should occur. Multiple gestations should not be cared for by non-physician providers or by family physicians. Referral to a maternal-fetal medicine unit is recommended.

Sonographic examination of twins. From first trimester to delivery of second fetus

Sonographic examination is essential in the diagnosis and management of twin gestation. It assists in determining the zygosity, assessing fetal anomalies, and is integral to amniocentesis if it is necessary, determining the growth and ruling out discordance, and in intrapartum management. The management of uncommon complications with twins also requires ultrasonic survey. Considering that the incidence of multiple gestation is increasing, it would be prudent to become familiar with the use and benefit of ultrasound with twins.