Gestation length varies among populations of common geckos on the South Island of New Zealand, from annual reproductive cycles at a relatively warm site at Alexandra to biennial cycles at a cool site at Macraes. We compared the effect of warm and cool thermal regimes on the length and success of gestation between geckos from Macraes and Alexandra. To assess the likelihood of geckos experiencing these temperatures in the wild, we compared the laboratory thermal regimes with data available for field microhabitat temperatures, body temperatures (T_b), and selected temperatures (T_sel) of pregnant females. Temperature had a profound effect on the length and success of pregnancy; geckos from both populations under the cool regime had longer pregnancies that were less successful in producing developed neonates than geckos under the warm regime. Though no viable young were produced under the cool regime, there were qualitative differences in outcome of pregnancies between populations; all geckos from Alexandra (warm site) aborted pregnancies early on, whereas several from Macraes (cool site) produced fully developed embryos after extremely long gestation periods (up to 440 days). Under the warm regime, populations appeared to differ in the rate of embryogenesis and in neonatal growth rate (both in g/day, with individuals from the Macraes population having the highest rates). Though temperatures of both thermal regimes overlapped with common microhabitat temperatures for both populations in most seasons of embryogenesis, maximum field T_b and T_sel for pregnant females were generally higher than in the laboratory regimes, at least for certain times of day in warm weather. The prolonged embryogenesis exhibited by geckos under the warm regime and the poor success of pregnancy under the cool regime demonstrate the necessity of thermoregulation by pregnant females for successful reproduction, particularly at Macraes. Our study describes complex but significant plasticity in the thermal sensitivity of female reproduction in a viviparous gekkonid lizard.