After parturition (Day 0 postpartum), mother rats were mated with either a fertile male (lactating pregnant rats) or a vasectomized male (lactating pseudopregnant rats). The length of lactation was limited to 7, 12 or 15 days (7 day, 12 day or 15 day group). In lactating pregnant rats, pregnancy successfully followed after removal of pups in all of the 7 day group, half of the 12 day group but none of the 15 day group. Once gestation started, it lasted for 16-18 days after removal of pups indicating that the day of the removal of pups corresponded to Days 4-6 of normal pregnancy. Elevated progesterone levels were maintained throughout the gestational period. In lactating pseudopregnant rats, however, the functional corpora lutea were always maintained for 19 days regardless of whether there were different lengths of concurrent lactation, which suggested that these corpora lutea were maintained by essentially the same mechanism as in "extended pseudopregnancy."

These observations suggest that the prolonged gestation in lactating pregnant rats is maintained solely by the corpora lutea formed at postpartum ovulation. Since the maintenance of these corpora lutea with the aid of hypophysial prolactin is limited to 19 days, the entry of placental luteotropin expected to occur 5-7 days after removal of pups appears to be crucial for the completion of prolonged gestation.