||In rats, birth timing is affected by changes in the light schedule until the middle of the pregnancy period. This phenomenon can be used to control birth timing in the animal industry and/or clinical fields. However, changes in the light schedule until the middle of the pregnancy period can damage the fetus by affecting the development of the major organs. Thus, we compared birth timing in mice kept under a 12-h light/12-h darkness schedule (L/D) throughout pregnancy with that of mice kept under a light schedule that changed from L/D to constant light (L/L) or constant darkness (D/D) from day 17.5 of pregnancy, the latter phase of the pregnancy period. On average, the pregnancy period was longer in D/D mice (19.9 days) than L/L or L/D mice (19.5 and 19.3 days, respectively, P < 0.05), confirming that light schedule affects birth timing. The average number of newborns was the same in L/L, L/D, and D/D mice (7.5, 7.8, and 7.9, respectively), but the average newborn weight of L/L mice (1.3 g) was lower than that of L/D and D/D mice (both 1.4 g, P < 0.05), indicating that constant light has detrimental effects on fetus growth. However, the percentage of dead newborns was the same between L/L, L/D, and D/D mice (11.1, 10.6, and 3.6%, respectively). The serum progesterone level on day 18.5 of pregnancy in L/D mice was 42.8 ng/ml, lower (P < 0.05) than that of D/D mice (65.3 ng/ml), suggesting that light schedule affects luteolysis. The average pregnancy period of mice lacking a circadian clock kept under D/D conditions from day 17.5 of pregnancy (KO D/D) (20.3 days) was delayed compared with wild-type (WT) D/D mice (P < 0.05). However, the average number of newborns, percentage of births with dead pups, and weight per newborn of KO D/D mice (7.6, 3.6%, and 1.4 g, respectively) were the same as WT mice kept under D/D conditions. A direct effect of the circadian clock on the mechanism(s) regulating birth timing was questionable, as the lighter average weight per KO fetus (0.6 g) versus WT fetus (0.7 g) on day 17.5 of pregnancy might have caused the delay in birth. The range of birth timing in KO D/D mice was the same as that of WT D/D mice, indicating that the circadian clock does not concentrate births at one time.