Global temperatures reached exceptionally high levels in 2023. More intense and frequent temperature extremes cause the greatest mortality of all extreme weather events. Pregnant and postpartum women, infants and young children are vulnerable to heat, but are not specifically targeted by heat health warning systems (HHWS). When pregnant women are exposed to heat, adverse health outcomes can occur, such as placental abruption, preterm birth, stillbirth, miscarriages, and fetal growth restrictions. Within the framework of the EU-funded HIGH Horizons project, our study aims to identify heat indices and respective heat health risk thresholds to be integrated into a HHWS, a mobile application, to alert impending heat health risks, raise awareness and provide advice to protect vulnerable groups of pregnant and postpartum women, infants and young children, and health workers taking care of them.