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.

中文翻译：

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改变妊娠晚期的光照时间表会改变小鼠的出生时间

在大鼠中，出生时间受到光照计划变化的影响，直到怀孕中期。这种现象可用于控制动物工业和/或临床领域的出生时间。然而，直到怀孕中期的光照时间表的变化会通过影响主要器官的发育来损害胎儿。因此，我们比较了在整个怀孕期间保持在 12 小时光照/12 小时黑暗计划 (L/D) 下的小鼠的出生时间与保持在从 L/D 变为恒定光照 (L/L ) 或从怀孕第 17.5 天开始的持续黑暗 (D/D)，即怀孕期的后期。平均而言，D/D 小鼠的妊娠期（19.9 天）长于 L/L 或 L/D 小鼠（分别为 19.5 和 19.3 天，P < 0.05），证实了光照时间会影响出生时间。L/L、L/D 和 D/D 小鼠的平均新生儿数相同（分别为 7.5、7.8 和 7.9），但 L/L 小鼠的平均新生儿体重（1.3 g）低于L/D 和 D/D 小鼠（均为 1.4 g，P < 0.05），表明恒定光照对胎儿生长有不利影响。然而，L/L、L/D 和 D/D 小鼠的死亡新生儿百分比相同（分别为 11.1、10.6 和 3.6%）。L/D 小鼠妊娠第 18.5 天的血清黄体酮水平为 42.8 ng/ml，低于（P < 0.05）D/D 小鼠（65.3 ng/ml），表明光照计划影响黄体溶解。与野生型 (WT) D/D 小鼠相比，从怀孕第 17.5 天 (KO D/D)（20.3 天）开始，缺乏昼夜节律时钟的小鼠的平均妊娠期延迟（P < 0.05） . 然而，平均新生儿数，死亡幼崽的出生百分比和 KO D/D 小鼠每个新生儿的体重（分别为 7.6、3.6% 和 1.4 g）与保持在 D/D 条件下的 WT 小鼠相同。生物钟对调节出生时间的机制的直接影响是有问题的，因为在怀孕第 17.5 天，每个 KO 胎儿（0.6 克）与 WT 胎儿（0.7 克）的平均体重较轻可能导致了出生延迟. KO D/D 小鼠的出生时间范围与 WT D/D 小鼠的出生时间范围相同，表明生物钟不会一次集中出生。6 g) 与 WT 胎儿 (0.7 g) 在怀孕第 17.5 天可能导致出生延迟。KO D/D 小鼠的出生时间范围与 WT D/D 小鼠的出生时间范围相同，表明生物钟不会一次集中出生。6 g) 与 WT 胎儿 (0.7 g) 在怀孕第 17.5 天可能导致出生延迟。KO D/D 小鼠的出生时间范围与 WT D/D 小鼠的出生时间范围相同，表明生物钟不会一次集中出生。