Fetal tissues such as adrenal glands and the suprachiasmatic nucleus (SCN) may be entrained by rhythms of maternal feeding schedules, core body temperature and melatonin. Entraining signals, including melatonin and glucocorticoids, may cross the placental barrier to entrain, modulate or aid the development of fetal circadian rhythms among brain and peripheral tissues. ANS, autonomic nervous system; RHT, retinohypothalamic tract.