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Studies performed over the past 20 years have revealed that mother rodents can provide photoperiod information to their developing fetuses. In adult mammals, the pattern of pineal melatonin secretion changes in relation to changes in day length, and the melatonin pattern is a key part of the photoperiodic mechanism. Melatonin crosses the placenta, and fetal rodents can respond to the maternal melatonin rhythm. Thus, the mother's melatonin rhythm provides day-length information to the fetus, and this information is used, along with photoperiod information that is obtained after birth, to influence juvenile development. The transfer of photoperiod information from mother to fetus may be part of an adaptive system. When young are born early in the spring or summer breeding season, the increase in day length between the times of fetal and postnatal life results in rapid reproductive maturation, allowing these early-born animals to reproduce later during the same breeding season. In contrast, for young born late in the breeding season, the decrease in photoperiod between fetal and postnatal life results in delayed maturation of the gonads, and reproduction is delayed until the beginning of the next year's breeding season.