Maternal outcomes at mid-altitudes in anemic pregnant women: Evidence from the WOMAN-2 trial

Background High residential altitude is associated with adverse pregnancy outcomes, yet most research has focused on elevations above 2500 meters. There is limited evidence on how altitude impacts maternal health below this threshold, where 99% of the global population resides. We investigated the effect of mid-altitudes on maternal outcomes among women with moderate to severe anemia who delivered in hospitals in four sub-Saharan and Asian countries. Methods We analysed data from the World Maternal Antifibrinolytic-2 (WOMAN-2) trial, which recruited anaemic women admitted for vaginal delivery across 42 hospital sites in Nigeria, Pakistan, Tanzania, and Zambia. We compared women in mid-altitudes (1,000–2,499 m) with those at low-altitudes (< 1,000 m) using postpartum blood loss as the primary outcome. The key secondary outcome was clinical postpartum haemorrhage (PPH). Additional secondary outcomes included prenatal maternal health indicators (gestational age at labor onset, hypertensive disorders of pregnancy, baseline hemoglobin, severe anemia, and mean arterial pressure [MAP]), as well as intrapartum intervention indicators (assisted delivery, use of medical pain control, and use of oxytocin in labor). We used STATA version 18 for data analysis. We used classical logistic and modified Poisson regressions with cluster-robust standard errors to estimate the odds and risk ratios (RR) for dichotomous outcomes and linear regression with cluster-robust variance to estimate the β coefficients for the continuous outcomes. Results Of 15,068 women, 4801 (31.9%) delivered at mid-altitude and 10,267 (68.1%) at low-altitudes. Delivery in mid-altitudes did not independently affect postpartum blood loss (β = 37.89 mL; 95% CI: −26.83–102.62), p = 0.243. Mid-altitude delivery compared to low-altitude was independently associated with a 22% lower risk of preterm delivery (aRR = 0.78; 95%CI: 0.73–0.83), p < 0.001, and a 26% lower risk of severe anaemia (aRR = 0.74; 95% CI: 0.68–0.82), p < 0.001. The use of medical pain control was 2.2 times higher (aRR = 2.20; 95% CI: 2.02–2.40), p < 0.001, and the Odds of assisted delivery were 46% more likely (aOR = 1.46; 95% CI: 1.20–1.78) at mid-altitudes compared to low-altitudes. Conclusion Delivery at mid-altitudes was not independently associated with postpartum blood loss, but generally exhibited more favorable prenatal maternal health indicators and more intrapartum interventions than those delivering at low-altitudes.