EP08.24: Fetal biometry growth from first to second trimester and prediction of stillbirth risk

Fetal growth restriction (FGR) is the most important risk factor for stillbirth. The aim of the study was to investigate if smaller than expected biometric measurements in second trimester might help to predict stillbirth risk. We identified singleton pregnancies with delivery during the years 2014-2021 at 22+0 gestational weeks or later in the Swedish Pregnancy Register. We included 377,563 pregnancies with a dating ultrasound (6+0-13+6) and a second trimester ultrasound (16+0-21+6 weeks), and retrieved information of biometric measurements (biparietal diameter [BPD], abdominal circumference [AC], and femur length [FL]. Adjusted relative risks (aRR) with 95% confidence interval (CI) for stillbirth were calculated for fetuses with deviant biometry (<5th centile for each biometric measurement) in second trimester, and adjusted for maternal country of birth, parity, in vitro fertilization, smoking, body mass index and maternal age. Fetuses with BPD, AC, and FL ≥5th centile were treated as reference category in all analyses. The reference category included 84% of the fetuses. Short FL in second trimester (with or without small BPD and/or AC) occurred in 2.3% of the pregnancies, and was associated with increased risk of stillbirth; aRR (95% CI) 2.20 (1.61-3.00). Normal FL with small BPD and/or AC was not associated with increased risk of stillbirth. The highest risk of stillbirth was seen in fetuses with normal BPD but small AC and FL, and fetuses with all biometry small; aRR (95% CI) 2.78 (1.44-5.34) and 2.40 (1.14-5.06), respectively. The risk of stillbirth is increased in fetuses with short FL in the second trimester, but not in fetuses with normal FL but small BPD and/or AC. Short FL in second trimester may be a clinically relevant marker for early placental dysfunction, especially in coexistence with small AC. Further research is needed to evaluate if short FL in second trimester gives grounds for increased fetal growth surveillance.