Photoperiod: an important regulator of plasma prolactin concentration in fetal lambs during late gestation

Measurement of prolactin (PRL) at 120-140 days gestation in chronically-cannulated fetal lambs and their mothers during hormonally-extended breeding seasons shows that photoperiod is a major determinant of PRL secretion in the fetal lamb as well as in its mother. About 80% of the variance between fetuses in plasma PRL concentration is associated with variation in ambient photoperiod. The magnitude of this effect far exceeds the range of previously reported diurnal or ontogenetic changes in PRL in the fetal lamb.