The effects of luteinizing hormone releasing hormone (LHRH) in pregnant rats. 2. Prenidatory effects and delayed parturition.

During the prenidatory period, pregnancy is terminated in ratsby doses of 150 �g twice daily (b.i.d.) of LHRH from Days 1-7; a partial effect is seen at 50 Mg. Treatment with doses as large as 500 Mg b.i.d. of LHRH over Days 1-3 did not result in termination of pregnancy, but the fetuses were smaller in size than controls at autopsy on Day 15. When injected twice on either Day 3 or 4, 500 �g of LHRH markedly depressed fetal size; in contrast, the same treatment on Days 1, 2, 5, 6 or 7 was without effect. The depressions in fetal size were explainable on the basis of a 2-3 day delay of implantation following LHRH (500 Mg b.i.d.) on Day 3 or 4, which was secondary to retardation of tubal transport (Days 3 and 4) and slowed cleavage (Day 3). Estradiol given on Day 4, but not on Day 3, normalized the effects of LHRH treatment on Day 3 or 4, suggesting that the delay of nidation resulted from estrogen (ovarian) failure. Progesterone had no effect on the delayed implantation. Delayed nidation was followed by postponement of parturition, albeit litters appeared normal.