The effect of medroxyprogesterone acetate on premature labor in the sheep.

The ability of medroxyprogesterone acetate to delay premature delivery was investigated in nine chronically catheterized pregnant sheep (125 to 130 days' gestation). Catheters were placed for measurement of intrauterine pressure and uterine vein concentrations of progesterone, 17 beta-estradiol, and 15-keto-13,14-dihydroxyprostaglandin F2a. Premature parturition was induced in all animals by infusion of dexamethasone (1 mg/24 hours) to the fetus. Three ewes served as controls, three ewes received oral medroxyprogesterone acetate (5 mg/kg/day) 48 hours prior to the start of and during dexamethasone (early medroxyprogesterone acetate), and the remaining three animals received oral medroxyprogesterone acetate 12 hours after the onset of dexamethasone infusion (late medroxyprogesterone acetate). The lengths of time from dexamethasone until the onset of labor and from dexamethasone until delivery were compared, and were significantly longer for each group compared to control times. Of all endocrine events analyzed, the fall in plasma progesterone was the most consistent in all groups, showing that animals with rapid falls in progesterone levels (control group) tended to deliver earlier. These results demonstrate that medroxyprogesterone acetate can delay premature parturition in the sheep. Also medroxyprogesterone acetate appears to delay the endocrinologic events that normally occur at this time.