Termination of pregnancy after accelerated lactation in the rat. II. Relationship to number of young, day of pregnancy and length of nursing.

Pregnancy is terminated in rats that have mated at the postpartum oestrus if they are allowed to suckle a large litter after implantation. For consistent termination, the number of sucking young must be at least nine, the accelerated lactation must be begun before Day 10 of pregnancy, and must be continued for at least 2 days.