Timing of termination of nocturnal prolactin surges in pregnant rats as determined by the number of fetuses

In female rats bearing an average litter of 10 ± 0.28 fetuses, pituitary prolactin is secreted in a pattern of 2 daily surges during the first 10 days of pregnancy only. When the number of fetuses was experimentally reduced, prolactin surges continued for a longer period and pregnancy was correspondingly lengthened. The relationship found between a reduced number of fetuses, and delay in termination of prolactin surges further supports the view that, during pregnancy, rat placental lactogen exerts a major influence in inhibiting secretion of prolactin by the maternal pituitary gland. The constant relationship found in all groups between termination of nocturnal surges of prolactin and parturition indicates that it is the interval between implantation and termination of prolactin surges which is prolonged in female rats with small litters.