Climate Condition during embryonic development can exert profound and long-term effects on many types of Organisms, but most previous research on this topic has focussed on endothermic vertebrates. Although viviparity in ectothermic taxa allows the reproducing female to buffer ambient thermal variation for her development offspring, even an actively thermoregulating female maybe unable to provide optimal incubation regimes in severe weather conditions. Examination to the extend to which fluctuations in natural thermal conditions during pregnancy affect reproduction in a temperate viviparous snake, the Asp Viper (Vipera aspis). Data gathered from a long term field study demonstrated that ambient thermal conditions influenced (1) Female Body temperatures (2) Gestation length, embryo viability and offspring phenotypes. Interestingly, thermal conditions during each of the three months of the gestation affected different aspects of reproduction. Hotter weather early Gestation (June), Increased ventral scale count (=no: of Body Segments) of neonates; hotter weather mid-gestation (July) hastened development and thus the date of parturition, and hotter weather late in gestation(August) reduced the incidence of still born neonates. The population studied is close to the northern limit of the species range, and embryonic thermal requirements may prevent Vipera aspis from extending into cooler conditions further north.