OP05.03: First trimester biparietal diameter for pregnancy dating

To evaluate the accuracy of measurement of fetal biparietal diameter (BPD) at 11-13 weeks, compared to the measurement of Crown–rump length (CRL) for pregnancy dating. Retrospective, cohort study performed in Spain, UK, Belgium and Bulgaria. Population: women with singleton, non-malformed fetus/neonate, CRL and BPD measurements at 11-13 wks and pregnancy outcome. We developed a formula for pregnancy dating based on BPD using pregnancies conceived by in-vitro fertilisation (IVF), by a simple linear regression. We validated this formula and compared it with the most commonly used ones (Robinson CRL-based and Kustermann BPD-based) through the Euclidean distance, relative absolute error (RAE), and mean squared error (MSE). We also examined the rate of labour induction for prolonged pregnancy based on the dating of each formula. 35810 participants were included, comprising 33541 (93.7%) conceiving spontaneously and 2269 (6.3%) by IVF. Internal validation in IVF pregnancies: the newly developed formula showed no statistically significant difference compared to the real gestational age (GA) estimated by conception date [mean difference of 0.88 days (95%CI 0.69-1.06)]. The mean difference of the Kustermann BPD-based formula was 0.62 days (95%CI 0.45,0.80) and the mean difference of the Robinson CRL-based formula was 1.83 days (95%CI 1.71,1.95). External validation in spontaneous pregnancies: both our formula and the Kustermann's resulted in underestimated GA, with statistically significant mean differences of -0.69 days (95%CI -0.72,-0.66) and -0.92 days (95%CI -0.95,-0.89), respectively, using dating from Robinson's formula as the reference for ‘true’ GA. The largest differences with Robinson's dating were observed between 11 and 12 weeks. Dating the pregnancy with Robinson's formula led to 8.1% of pregnancies requiring induction after 41+3 weeks, as compared to 7.7% (p = 0.26) and 7.0% (p =< 0.001) with our formula and Kustermann's, respectively. Pregnancy dating based on ultrasound measurement of fetal BPD at 11-13 weeks of gestation is a reliable alternative to dating based on CRL.