Introduction of a new crop of corpora lutea during pregnancy and its effect on luteal function and parturition in the rat

To analyze the control mechanism of luteal function during pregnancy, a new crop of corpora lutea was introduced in pregnant rats from Day 4 to Day 19 of pregnancy (Day 1, first day of leukocytic smear) with a single i.v. injection of 20 μg LH. Between Days 12-16, pretreatment with 30 IU PMSG before the LH treatment was needed for ovulation to occur. Pregnancy was not interrupted by these treatments. Induction of ovulation before Day 10 had an effect on peripheral progesterone and 20α-dihydroprogesterone levels but had no effect on the duration of gestation. Hypophysectomy of these rats on Day 11 but not on Day 19 prevented the onset of parturition and prolonged gestation. Formation of new corpora lutea later than Day 12 also delayed parturition which then occurred between Days 25-27 and usually resulted in the death of either the mothers or fetuses. Elevated progesterone levels were observed during this extended pregnancy. These levels appeared to be maintained by placental luteotropin, since removal of the conceptus was followed by an immediate decrease in progesterone concentrations. Thus, a particular period was defined: before Day 10 of pregnancy when induction of ovulation was possible with LH alone luteolysis and parturition occurred at term; after Day 12, however, pretreatment with PMSG was needed for ovulation to occur and the function of the corpora lutea was extended beyond term.