Maternal body mass index and risk of post-partum haemorrhage: A population-based study in primiparous women

Post-partum haemorrhage (PPH) is a leading cause of maternal morbidity and mortality, yet its relationship with body mass index (BMI) remains unclear.

To examine the association between BMI and PPH among primiparous women in Australia.

A population-based retrospective cohort study was conducted using Victorian Perinatal Data Collection registry (2010─2019). BMI data were available for 311,266 primiparous women. Post-partum haemorrhage was defined using two blood loss thresholds: ≥ 500 mL and ≥ 1000 mL. Logistic regression models were used.

The prevalence of overweight and obesity increased from 24.9% to 26.4% and 16.5-18.6%, respectively, while PPH rates rose from 25.5% to 31.5% (≥500 mL) and 2.6-6.4% (≥1000 mL). Women with overweight and obesity had higher odds of PPH compared to women with normal BMI: adjusted odds ratios (AORs) for PPH ≥ 500 mL were 1.11 (95% CI: 1.07-1.13) for overweight and 1.32 (95% CI: 1.29-1.36) for obesity. For PPH ≥ 1000 mL, AORs were 1.10 (95% CI: 1.05-1.15) and 1.30 (95% CI: 1.24-1.37), respectively. Subgroup analysis of vaginal births showed a stronger association between obese BMI and PPH ≥ 1000 mL. For caesarean births, BMI was not significantly associated with PPH ≥ 1000 mL.

Higher maternal BMI is significantly associated with PPH, particularly in vaginal births. Rising PPH rates over time align with increasing overweight and obesity prevalence, but cannot be fully explained by BMI alone. Enhanced focus on pre-pregnancy weight management and tailored maternity care could contribute to mitigating these risks and improving maternal health outcomes.