The effects of medroxyprogesterone acetate on the maintenance of pregnancy in ovariectomized rats and rabbits

Abstract

AbstractFetal survival, malformation rate, and fetal and placental weights were examined near term in control rabbits, rabbits treated on day 6 with 5 mg of medroxyprogesterone acetate (MPA), a long acting progestin, and rabbits treated with 5 mg of MPA on day 6 and ovariectomized on day 7. MPA alone had no apparent effect on fetal or placental development but MPA plus ovariectomy increased fetal weight by 19% and placental weight by 60%. Fetal and placental development were examined near term in rats given similar treatments. One milligram of MPA administered alone on day 7 had no apparent effect. However, in rats given 1 mg of MPA on day 6 and ovariectomized on day 7 no fetuses survived. In rats given 1 mg of MPA on day 11 and ovariectomized on day 12 only 4.7% of the fetuses survived and all were pressure malformed. In rats given 1 mg of MPA on day 11, ovariectomized on day 12, and supplemented daily with 0.5 μg of estradiol benzoate thereafter, 40% of the fetuses survived and none was malformed, but fetal weights were 22% and placental weights 15% below those of the controls. The underlying problem in the ovariectomized rats supplemented with progestins alone appeared to be failure of myometrial growth. These results highlight the species differences in steroid hormone requirements for fetal, placental, and myometrial development during pregnancy.