Climate change has severe and wide-ranging health impacts, especially for vulnerable groups. Despite growing evidence of heat-associated adverse maternal and neonatal health outcomes, there remains a lack of synthesis quantifying associations and identifying specific risk periods. We systematically reviewed the literature on heat impacts on maternal, fetal and neonatal health and quantified impacts through meta-analyses. We found 198 studies across 66 countries, predominantly high income (63.3%) and temperate climate zones (40.1%), and 23 outcomes. Results showed increased odds of preterm birth of 1.04 (95% confidence interval (CI) = 1.03, 1.06; n = 12) per 1 °C increase in heat exposure and 1.26 (95% CI = 1.08, 1.47; n = 10) during heat waves. Similarly, high heat exposure increased the risk for stillbirths (odds ratio (OR) = 1.13 (95% CI = 0.95, 1.34; n = 9)), congenital anomalies (OR = 1.48 (95% CI = 1.16, 1.88; n = 6)) and gestational diabetes mellitus (OR = 1.28 (95% CI = 1.05, 1.74; n = 4)). The odds of any obstetric complication increased by 1.25 (95% CI = 1.09, 1.42; n = 11) during heat waves. Patterns in susceptibility windows varied by condition. The findings were limited by heterogeneity in exposure metrics and study designs. The systematic review demonstrated that escalating heat exposure poses a major threat to maternal and neonatal health, highlighting research priorities, guiding the selection and monitoring of heat-health indicators and emphasizing the need to prioritize maternal and neonatal health in national climate health programs.