For all climatic regions, mortality due to cold exceeds mortality due to heat. We examine whether cold-related mortality in adulthood varies positively with unusually benign ambient temperature during gestation, using data on over 13,500 Swedes from the Uppsala Birth Cohort Study born in 1915-1929 and followed until 2003. We link daily thermometer temperatures in Uppsala (1914 to 2002) to subjects, from their estimated date of conception onwards. We estimate survival models with time-varying explanatory variables, focusing on the two leading causes of cold-related death in adulthood: ischaemic heart disease (IHD) and stroke. An increase in the prevalence of warm temperatures during gestation leads to a significantly higher rate of mortality due to cold-related IHD. However, we do not find such a relation for cold-related stroke mortality. Additional analyses show that birthweight percentile or gestational age do not mediate discovered findings. The IHD results indicate that ambient temperature during gestation – independent of birth month – modifies the relation between cold and adult mortality.