The circadian-gated timing of birth in rats: disruption by maternal SCN lesions or by removal of the fetal brain.

The roles of the maternal suprachiasmatic nuclei (SCN) and fetal brain in the circadian-gated timing of birth were studied in rats. The circadian gating of birth was shown by exposing different groups of dams to lighting cycles of opposite phase during pregnancy. Regardless of the phase of the prenatal lighting cycle, the time of birth was gated over a 36-h temporal window so that most births occurred during subjective day. Destruction of the maternal SCN eliminated the circadian gating; births occurred in a single distribution that peaked in the middle of the 36-h window. Removal of the fetal brain also disrupted the circadian gating of birth; dams of brain-aspirated fetuses no longer exhibited a daytime preference for births. These results show that the maternal SCN are necessary for the normal circadian gating of birth and are also consistent with a role for the fetal brain in this process.