Is short first-trimester crown-rump length associated with spontaneous preterm birth?

Objective To assess the association between first-trimester crown-rump length (CRL) and the risk of spontaneous preterm birth before 32 weeks' gestation. Methods We performed a matched case-control study of 129 women with spontaneous preterm birth at < 32 weeks' gestation (cases) and 129 women with term deliveries (controls) using data stored in the ultrasound and obstetric databases of our tertiary referral center. Cases and controls were individually matched based on maternal age, parity, history of preterm birth and medical indication for antenatal care. Fetal CRL measured between 8 + 0 and 13 + 6 weeks was expressed as multiples of the median (MoM) expected CRL, based on last menstrual period. We investigated the association between CRL-MoM and spontaneous preterm birth using logistic regression analysis. Results CRL-MoM was not associated with spontaneous preterm birth: odds ratio (OR) 1.10 (95% CI, 0.89-1.36) per 0.10 MoM increase in CRL. Timing of measurement did not influence the model (P = 0.59). This was confirmed when restricting the analysis to the 93 pairs with CRL measurements made between 10 + 0 and 13 + 6 weeks: OR for preterm birth 1.07 (95% CI, 0.83-1.37) per 0.10 MoM increase in CRL. Conclusion A short CRL in the first trimester is not associated with spontaneous preterm birth before 32 weeks' gestation, thus short CRL cannot be used to identify women at increased risk of preterm birth.