The effect of estrogen on fetal survival in progesterone-treated ovariectomized rats

In pregnant rats ovariectomized on Day 14 of pregnancy (sperm present=Day 1), the daily injection of 4 mg progesterone maintained normal pregnancy up to Day 17, but injured fetuses bearing hematomata on their extremities appeared very frequently thereafter, in spite of successive injections of progesterone. The addition of 0.2 microgram estradiol-17 beta to the daily dose of progesterone for two successive days between Days 15 and 18 of pregnancy prevented the occurrence of fetal injuries. Estradiol-17 beta was found to be more effective in preventing fetal damage than estrone or estriol. The results suggest that while progesterone is essential for the maintenance of pregnancy, estrogen is also necessary at the time when the conceptus elongates and becomes cylindrical.