Fetal biometry and maternal characteristics for second trimester prediction of stillbirth

Small fetal size is the most important risk factor for stillbirth. To our knowledge, no previous study has considered combinations of mid-trimester measures of fetal size as predictors of stillbirth risk. The aim of this cohort study was to assess whether small fetal measurements by mid-pregnancy ultrasound, in conjunction with maternal factors, can predict the risk of stillbirth.

In this historical cohort study, we identified 377 563 singleton pregnancies in the Swedish Pregnancy Register between 2014 and 2021, with delivery at ≥22⁺⁰ gestational weeks with a first trimester (6⁺⁰-13⁺⁶ weeks) dating ultrasound, and second trimester (16⁺⁰-21⁺⁶ weeks) measurements of biparietal diameter (BPD), abdominal circumference (AC), and femur length (FL). Relative risks (RR) with a 95% confidence interval (CI) for stillbirth were calculated for fetuses with any second-trimester biometry <5th centile, and adjusted for maternal characteristics. Odds ratios (ORs) with 95% CI were used to assess the individual contributions of fetal biometry and maternal characteristics to stillbirth risk. Significant factors were included in a multivariate prediction model, and the area under the receiver operating characteristic curve (AUC) was used to assess predictive performance.

Short second-trimester FL was associated with increased risk of stillbirth, especially when combined with small AC or both small BPD and AC (aRR [95% CI] 2.78 [1.44-5.34] and 2.40 [1.14-5.06], respectively), whereas normal FL with small BPD and/or AC was not. In the multivariable model, short FL in combination with small AC was the strongest contributor to stillbirth risk assessment. The AUC of the prediction model, including both maternal characteristics, small AC, and short FL, was 0.623 for stillbirth in all pregnancies, 0.635 for term stillbirth, and 0.591 for preterm stillbirth.

We found evidence of increased risk of stillbirth among fetuses with short FL, especially in combination with small AC, but not with normal FL, regardless of other small fetal dimensions. However, maternal characteristics and mid-gestational biometry perform poorly in predicting stillbirth.