The effect of progesterone on the early development of the rabbit embryo

The effects of subcontraceptive levels of progesterone administered near the time of ovulation on embryo development in vitro; embryo development in the treated animals and following transfer to control recipients were studied. One group of does, designated as P₁, received 0.5 mg of progesterone on Day 2 and 1.0 mg on Days 1 and 0, considering Day 0 as the day an ovulating dose of LH was given. Another group, P₂, received the same dose of progesterone starting on Day 1, and a third group served as controls. Approximately 2000 eggs and blastocysts were collected from 122 does, and examined, cultured, or transferred to recipients. Ovulation and fertilization rates were unaffected by treatment. Four days after giving LH, the percentages of embryos that had developed into blastocysts in the P₁, P₂, and control groups were 58.6, 84.8, and 82.8, respectively, indicating that the P₁ progesterone treatment tended to suppress development of zygotes into blastocysts. Four day blastocysts from P₁, P₂, and control does transferred to control recipients yielded 4.1, 20.4, and 44.4% implants at Day 12 of pregnancy. Corresponding values for implantations in does without transfers were 8.6, 59.3, and 97.0%. Thus, there was a dramatic reduction in viable embryos between Days 4 and 12. Since this occurred regardless of whether or not the embryos continued to reside in the uteri of treated does, it is clear that the morphologically normal appearing blastocysts had been damaged prior to Day 4. Recovery data suggested that progesterone may have altered embryo transport and thereby led to asynchrony between the developing embryo and its environment. In all treatments embryos which implanted successfully developed normally and were equivalent to controls in postnatal development.