Abstract
SUMMARY: Mice were ovariectomized on day 4 of pregnancy and injected daily with either progesterone or oil vehicle (day 1 = the day of vaginal plug). Blastocysts were recovered on day 10, 20 or 30, counted and transferred to ovariectomized recipients. Implantation was induced and pregnancy maintained by daily injections of oestradiol and progesterone. Viability of the blastocysts was assessed by determining the percentage that developed into normal foetuses. A similar number of eggs was recovered from both groups at each time-interval; thus progesterone did not influence the retention of blastocysts by ovariectomized mice. In contrast, progesterone did influence the viability of the embryos. The percentage of blastocysts developing normally from the oil-treated animals was: 39% on day 10, 19% on day 20 and less than 1% on day 30. Corresponding percentages for the progesterone-treated animals were: 40% on day 10, 35% on day 20 and 35% on day 30.

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