Pregnant females of the viviparous lizard species Sceloporus jarrovi regulate their body temperatures at a lower level than do males or nonpregnant females. It has been suggested that such a shift in preferred body temperature during pregnancy reflects the presence of divergent optimal temperatures for the female and for development of her young; a pregnant female must compromise between these temperatures in order to maximize her fitness. We examine this hypothesis using a Leslie matrix model of life history that quantitatively predicts the mean body temperature that would optimally compromise between conflicting thermal optima for mother and embryos. The predictions of the model are in close agreement (0.4°C or less) with temperatures observed in the field. According to the model, a pregnant female maintaining the mean body temperature typical of males or nonpregnant females would have ≈13% lower fitness than a pregnant female maintaining the optimal temperature. This is mainly due to increased embryo mortality, but reduced growth and increased mortality of the female also contribute to the loss of fitness. In the model, the optimal mean body temperature depends on the precision of thermoregulation. Thermoregulation by gravid females in the field is imprecise, with a standard deviation of 1.4° during active thermoregulation. If females are (unrealistically) assumed to maintain a perfectly constant body temperature (i.e., a standard deviation of 0°), the optimal mean temperature in the model is ≈2° higher than the mean temperature of gravid females in the field. Although specifically designed to incorporate features of the physiology and life history of Sceloporus jarrovi, the model can be generalized and applied to other situations involving compromise among multiple optima.