Risk of stillbirth after 37 weeks in pregnancies complicated by small-for-gestational-age fetuses

The evidence for delivering small-for-gestational-age (SGA) fetuses at 37 weeks remains conflicting. We examined the risk of stillbirth per week of gestation beyond 37 weeks for pregnancies complicated by SGA.

Singleton pregnancies undergoing routine second trimester ultrasound from 1990-2009 were examined retrospectively. The risk of stillbirth per 10,000 ongoing SGA pregnancies with 95% confidence intervals (CIs) was calculated for each week of gestation ≥37 weeks. Using a life-table analysis with correction for censoring, conditional risks of stillbirth, cumulative risks of stillbirth per 10,000 ongoing SGA pregnancies and relative risks (RRs) were calculated with 95% CIs for each week of gestation.

Among 57,195 pregnancies meeting inclusion criteria the background risk of stillbirth was 56/10,000 (95% CI, 42.3-72.7) with stillbirth risk for SGA pregnancies of 251/10,000 (95% CI, 221.2-284.5). The risk of stillbirth after the 37th week was greater compared with pregnancies delivered in the 37th week (47/10,000, 95% CI, 34.6-62.5 vs 21/10,000, 95% CI, 13.0-32.1; RR, 2.2; 95% CI, 1.3-3.7). The cumulative risk of stillbirth rose from 28/10,000 ongoing SGA pregnancies at 37 weeks to 77/10,000 at 39 weeks (RR, 2.75; 95% CI, 1.79-4.2). Among pregnancies complicated by SGA <5% the cumulative risk of stillbirth at 38 weeks was significantly greater than the risk at 37 weeks (RR, 2.3; 95% CI, 1.4-3.8).

There is a significantly increased risk of stillbirth in pregnancies complicated by SGA delivered after the 37th week. Given these findings, we advocate a policy of delivery of SGA pregnancies 37-38 weeks.