The beneficial effect of progesterone on blastocyst survival in the uterus was shown in rats hypophysectomized on Day 1 of pregnancy. Treatment with progesterone (2 mg/day) on Day 1, Day 1 and Day 6, or Days 1 to 5 could maintain most blastocysts for only 5 days following the last injection; during the next 5 days, many disintegrated in the uterus. Daily injections of 2 mg progesterone, however, could maintain most blastocysts for as long as Day 53.
In hypophysectomized rats, the developmental potential of blastocysts deteriorated rapidly even though the rats received 2 mg progesterone daily. By Day 20, only 23% of blastocysts were capable of developing into full-term fetuses compared with 52% in rats ovariectomized on Day 4 and injected daily with 2 mg progesterone. By Day 52, none of the blastocysts from either group of animals developed to term although many still retained the ability to implant.
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