Is first-trimester crown-rump length associated with birthweight?

Objective To evaluate the relationship between first-trimester crown-rump length (CRL) and birthweight (BW) Z scores. Design Retrospective cohort study. Setting Two tertiary centres in France. Population Three hundred and seventeen pregnancies conceived through assisted reproductive techniques between April 2001 and December 2008. Methods We used CRL and worked forward to BW. Only pregnancies examined during the first trimester by an Fetal Medicine Foundation-certified operator were included. CRL was expressed as Z scores, and BW was transformed into Z scores by taking gestational age and gender into account. The influence of abnormal first-trimester CRL Z scores on BW was examined. Main outcome measures Weight and gestational age at birth. Results Birth weight was significantly greater in babies with larger CRL: BW Z scores (+/-SD) were) 0.36 (+/-1.05),) 0.27 (+/-0.97),) 0.10 (+/-1.04) and 0.13 (+/-0.96) in the first, second, third and fourth quartiles of CRL Z scores, respectively (P = 0.01). In contrast, there was no difference in gestational length according to the quartiles of the CRL Z scores. The CRL Z score was a significant predictor of the BW Z score (beta = 0.17, P = 0.001). After adjustment for maternal body mass index, a one-point increase in the first-trimester CRL Z score (i.e. 3.6 mm) was associated with a 39% decrease, 64% increase, 114% increase and 62% increase in the risk of having a BW below the 10th centile [odds ratio (OR), 0.61; 95% confidence intervals (95% CI), 0.39; 0.95; P = 0.03], above the 90th centile (OR, 1.64; 95% CI, 1.03; 2.60; P = 0.02), above the 95th centile (OR, 2.14; 95% CI, 1.25; 3.68; P = 0.006) and above 4000 g (OR, 1.62; 95% CI, 1.04; 2.51; P = 0.04), respectively. Conclusions Variations in BW may be partly explained by differences in growth trajectories that may express as early as the first trimester.