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

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BACKGROUND: Macrosomia, classically defined by an arbitrary birthweight threshold, is associated with an increased risk of postpartum hemorrhage. 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 postpartum hemorrhage. OBJECTIVE: We tested and quantified the independent association between birthweight and severe postpartum hemorrhage in a nationwide contemporary population of women with singleton birth at term. STUDY DESIGN: The data source was the French Enquête Nationale Périnatale (2021), a nationally representative population of all women who gave birth during 1 week in March 2021 in all maternity units in France (n=12,809). For this analysis, we included women with a live singleton birth ≥37 gestational weeks. Exposure was birthweight analyzed as a continuous variable. The primary outcome was severe postpartum hemorrhage, defined as a binary variable based on the presence of at least one of the following criteria: blood loss ≥1000 mL, embolization, surgical intervention, or blood transfusion. The association between birthweight and severe postpartum hemorrhage 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 postpartum hemorrhage. RESULTS: Among the 11,041 women included, the median birth weight was 3340 g and the 90th percentile was 3920 g. The overall incidence of severe postpartum hemorrhage was 2.7% (293/11,041; 95% confidence interval [2.4-3.0]) and increased from 1.5% for birth weights ≤2600 g to 10.3% for birth weights ≥4600 g. The association between birth weight and severe postpartum hemorrhage was linear, with an adjusted relative risk of severe postpartum hemorrhage of 1.29 (95% confidence interval 1.18-1.41) for each 200-g increase in birth weight for primiparous women and adjusted relative risk 1.09 (1.00-1.18, P value .043) for multiparous women. Birth weights >3900 g and 4000 g were associated with increased risk of severe postpartum hemorrhage for primiparous women (adjusted relative risk 2.62, 1.68-4.08; and 3.27, 2.02-5.32) and multiparous women (adjusted relative risk 1.70, 1.15-2.51; and 1.60, 1.03-2.51). CONCLUSION: In women with singleton birth at term, the risk of severe postpartum hemorrhage increased linearly with birth weight, and more notably for primiparous women: 29% per each 200-g increase in birth weight vs 9% for multiparous women. This result underlines the limitations of macrosomia definitions and suggests that birth weight, and the associated maternal risks, should be considered along the entire weight distribution, rather than systematically dichotomized above an arbitrary threshold, in both research and clinical practice.

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