Does infant birthweight percentile identify mothers at risk of severe morbidity? A Canadian population-based cohort...

There is a reverse J-shaped relation between newborn weight percentile and risk of perinatal mortality. Perinatal mortality itself is associated with severe maternal morbidity and mortality (SMM-M) around the index pregnancy, likely because the two share common etiologies, including placental dysfunction. We evaluated an infant's birthweight percentile and risk of its mother experiencing SMM-M.

This population-based cohort study was completed within a universal healthcare system in Ontario, Canada. Included were 2,203,490 singleton livebirths between 2002 and 2020. The study exposure was infant birthweight percentile for gestational age and sex. The 25th to 75th percentile served as the referent. The main outcome was SMM-M arising from 23 week's gestation up to 42 days postpartum. Multivariable modified Poisson regression generated relative risks (aRRs) and 95% confidence intervals (CI), adjusted for maternal age, income, rurality, pre-existing diabetes and hypertension.

A J-shaped relation was seen between birthweight and risk of SMM-M. Relative to the 25th to 75th (15.0 per 1000 livebirths), the aRR of SMM-M was 1.27 (95% CI 1.21, 1.32) at 5th to < 10th, 1.40 (95% CI 1.28, 1.53) at 2nd to < 3rd, and 1.48 (95% CI 1.36, 1.62) at < 1st birthweight percentile. At higher birthweights, the aRR was 1.16 (95% CI 1.11, 1.21) at 90th to < 95th, 1.24 (95% CI 1.13, 1.36) at 95th to < 96th, and 1.73 (95% CI 1.60, 1.87) at > 99th percentile.

There is a J-shaped relation between infant birthweight and risk of its mother experiencing SMM-M, likely due to shared risk factors and a common pathogenesis.