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Transformation of endometrium and fertility in late stages of pseudopregnancy in the rabbit

Summary. Ovulation was induced in rabbits between Days 14 and 18 of pseudopregnancy by an intravenous injection of hCG. Induction of ovulation from Day 16 onwards led to normal progestational endometrial transformation. In rabbits injected on Day 14 or 15, a normal preimplantation endometrial morphology developed, but not earlier than 7 days after hCG (Day 14/15 +7). Uteroglobin secretion was advanced during the second pseudopregnancy. After mating or artificial insemination, fertility was greatest on Day 18 of pseudopregnancy. Conception failed on Day 14 and embryo transfers were unsuccessful on Day 14 +1. Transfers performed on Day 14 +3, however, led to implantation and offspring, even though endometrial morphology did not correspond to the normal Day 3 preimplantational morphology at the time of transfer.

We conclude that (1) endometrial transformation typical of normal pseudopregnancy can be induced by ovulation during the regeneration phase of pseudopregnancy from Day 16 onwards; (2) fertilization and implantation can be achieved as early as Day 15 of pseudopregnancy; (3) an oestrous period with high mating activity and fertility occurs about 3 days later; and (4) Day 14 after hCG represents a limited time of functional change from pseudopregnancy to a fertile uterine cycle in the rabbit.