Growth discrepancy in twins in the first trimester of pregnancy

Objective The prevalence and significance of intertwin growth discrepancy in the first trimester of pregnancy are controversial. The aim of this study was to refine the incidence and outcome of this discrepancy in relation to dating of the pregnancy and other biometric parameters. Methods This prospective study of twin pregnancies initially evaluated at 11-14 weeks' gestation was conducted between 2001 and 2004. Differences in crown-rump length (CRL) and estimated gestational age (GA) were calculated for each twin pair and their distribution was analyzed according to chorionicity and mode of conception (spontaneous or following assisted reproductive technologies). CRL-based GA and actual GA were compared. Other biometric parameters were also compared for each twin pair. The distribution of discrepancies was analyzed according to pregnancy outcome. Results A total of 182 twin pregnancies was included. The mean +/- SD discrepancy in CRL was 3.4 +/- 3.18 mm or 5.1 +/- 4.69% and the 95(th) centile was 9.8 mm or 14.3%. There was no influence of chorionicity (P = 0.44), mode of conception (P = 0.18) and no relation with outcome (P = 0.54). In conceptions resulting from assisted technology, the measured CRL of the smaller twin was closer to the actual GA. Cephalic and abdominal biometric measurements were significantly greater in the twin with the smaller CRL (P < 0.05). The two cases with a discrepancy > 15 mm were affected by trisomy 18 and triploidy, respectively. Conclusions CRL discrepancy in twin pregnancies in the first trimester is a frequent finding. Discrepancy > 95(th) centile indicates major growth delay of one twin, which could indicate the presence of aneuploidy. For milder degrees of discordance the CRL of the smaller fetus is a more accurate estimate of the actual GA. Copyright (c) 2005 ISUOG. Published by John Wiley & Sons, Ltd.