Prospective risk of stillbirth according to fetal size at term

Fetal growth and size are associated with stillbirth risk. We wanted to investigate the week by week predictive value of fetal size at term on the prospective risk of stillbirth.

Anonymised database from the UK GAP program, with 2,094,702 term (≥37 weeks) deliveries including 4,670 stillbirths. Prospective stillbirth risk was defined as fetal death in the current week divided by total undelivered pregnancies. The data were analysed in five centile bands for stillbirth risk and risk ratios (RR) for 38, 39, 40 and 41+ weeks, using appropriate for gestational age (AGA) at 37 weeks as reference.

Baseline stillbirth risk at 37 weeks was 0.29 per 1,000 and rose to 1.39 at 41+ weeks, with RR increasing to 4.8. In the 3<10 centile band risk rose from 0.72 to 2.43 over the same period, with RR increasing from 2.5 to 8.4. In the <3 centile group, the stillbirth risk rose from 1.62 to 6.16 (RR (5.6 to 21.2). In the 97>90 centile band, risk increased from 0.40 to 1.50 (RR 1.4 to 5.2) and for >97 centiles, it rose from 0.80 to 2.13 (RR 2.8-7.3).

Prospective risk of fetal death at term is related to fetal size. The information provided, together with other considerations and results of investigations where available, will help clinicians to determine the advice to give to mothers about the best timing for delivery, balancing the baby's risk of compromise with iatrogenic sequelae of early delivery.