Live birth is an evolutionary innovation that has enabled reptiles to colonise environments that are normally hostile for developing embryos. While the benefits of live bearing are undisputed, transitions from egg laying to live birth should be constrained since adult lizards and snakes typically have preferred body temperatures that exceed the upper lethal limit of embryos. However, live birth has evolved many times in lizards and snakes. Phylogenetic comparative analyses of 224 species revealed that transitions to live birth occur despite significant mismatches between the maternal and offspring thermal optima. We show that such mismatches are resolved by gravid females down-regulating their body temperature towards the thermal optimum of developing embryos. Importantly, this thermoregulatory behaviour evolved in ancestral egg laying species long before the evolutionary emergence of live bearing. Transitions to live bearing are then frequently followed by a reduction in preferred female body temperature that further eliminates conflicts over thermal optima between adults and embryos. In many of these lineages, females elevate their temperature when gravid – a behaviour that results in earlier birth and thus promotes offspring fitness. Maternal thermoregulatory behaviour therefore bypasses the constraints imposed by a slowly evolving thermal physiology and is likely to have been a key requirement for the repeated transitions to live birth across reptiles.