Characterization of the association between birth weight and severe postpartum hemorrhage in women with delivery at term

Macrosomia, classically defined by an arbitrary birthweight threshold, is associated with an increased risk of postpartum hemorrhage (PPH). However, some preliminary evidence suggests that lower birthweights may also be at increased risk. We hypothesized that birthweight, analyzed as a continuous variable, is significantly associated with the risk of severe postpartum hemorrhage, with the risk increasing not only at higher birthweights traditionally associated with macrosomia but also potentially at lower birthweights. This association was hypothesized to persist independently of confounding factors and may provide new insights into risk stratification and prevention strategies for severe PPH. We tested and quantified the independent association between birthweight and severe PPH in a nationwide contemporary population of women with singleton birth at term. The data source was the French Enquête Nationale Périnatale (ENP2021), a nationally representative population of all women who gave birth during 1 week in March 2021 in all maternity units in France (n=12809). For this analysis, we included women with a live singleton birth ≥37 gestational weeks. Exposure was birthweight analysed as a continuous variable. The primary outcome was severe PPH defined by a dichotomous variable with at least one of: blood loss≥1000 mL, embolization, surgery, or transfusion. The association between birthweight and severe PPH was tested by multivariate Poisson regression with stratification by parity because of significant interaction. Secondary analyses tested the association between macrosomia defined by thresholds of 3900 g (90th percentile of our study population) and 4000 g (the macrosomia threshold commonly used in the literature) and severe PPH. Among the 11041 women included, the median birthweight was 3340g and the 90th percentile was 3920g. The overall incidence of severe PPH was 2.7% (293/11,041; 95% CI [2.4 - 3.0]) and increased from 1.5% for birthweights ≤ 2600g to 10.3% for birthweights ≥ 4600 g. The association between birthweight and severe PPH was linear, with an adjusted relative risk (aRR) of severe PPH of 1.29 (95% confidence interval 1.18-1.41) for each 200-g increase in birthweight for primiparous women and aRR 1.09 (1.00-1.18, p 0.043) for multiparous women. Birthweights > 3900 g and 4000 g were associated with increased risk of severe PPH for primiparous women (aRR 2.62, 1.68-4.08; and 3.27, 2.02-5.32) and multiparous women (aRR 1.70, 1.15-2.51; and 1.60, 1.03-2.51). In women with singleton birth at term, the risk of severe PPH increased linearly with birthweight, and more notably for primiparous women: 29% per each 200g increase in birthweight versus 9% for multiparous women. This result underlines the limitations of macrosomia definitions and suggests that birthweight, and the associated maternal risks, should be considered along the entire weight distribution, rather than systematically dichotomised above an arbitrary threshold, in both research and clinical practice.