Risk of stillbirth at extremes of birth weight between 20 to 41 weeks gestation

To determine the risk of stillbirth between 20 to 41 weeks gestation, at highly detailed weight percentiles, including extreme degrees of small (SGA) and large (LGA) for gestational age birth weight.

We completed a population-based study of all births in Ontario, Canada between 2002 and 2007. We included 767, 016 liveborn and 4,697 stillborn singletons delivered between 20 and 41 weeks gestation. Smoothed birthweight percentile curves were generated for males and females, combining livebirths and stillbirths. Quantile regression was used to calculate sex-specific absolute birthweight differences and 95% confidence intervals (CI) between stillborns vs liveborns at various gestational ages. Logistic regression was used to calculate the odds ratios (OR) for stillbirth at various sex-specific birthweight percentiles, including <1st and ≥ 99th percentile. OR were adjusted for maternal age and parity.

At the 10th percentile, stillborns weighed significantly less than liveborns starting at 24 weeks gestation. By 32 weeks, this difference was 590 g (95% CI 430 to 750) for males and 551 g (95% CI 345 to 448) for females. A reverse J-shaped association was observed between birthweight percentile and risk of stillbirth across all gestational ages. Relative to the 40th to 60th percentile referent, the adjusted OR for stillbirth was 9.63 (95% CI 8.39 to 11.06) at a birth weight <1st percentile. At ≥ 99th percentile, the adjusted OR was 2.24 (95% CI 1.76 to 2.86). The risk of stillbirth at extreme birthweight percentiles was robustly observed across gestational ages.

Substantial birthweight differences exist between stillborns and newborns. As a possible hallmark of impending intrauterine death, severe SGA and LGA may each be potential targets for future stillbirth prevention initiatives.