Postpartum conception is a subject of great interest to rat owners. For example, some new rat owners are surprised by an unexpected litter. Their presumed same-sex pair of rats is actually a male-female pair! Such owners wish to know how quickly they should separate their rats to avoid another litter.
How soon can a female conceive another litter after giving birth?
The answer is soon! Females go into heat between about 10 and 24 hours after giving birth. Specifically, females go into heat on the first evening that is at least 10 hours after giving birth (Gilbert et al. 1985). This phenomenon of coming into heat shortly after giving birth is called postpartum estrus.
However, eggs fertilized during a female rat's postpartum estrus may not implant right away, due to a lactation-induced delay (Mantalenakis and Ketchel 1966). Female rats that conceive during their postpartum estrus may therefore go through an extended gestation of about 32 days (+-1 day) rather than the normal 21-22 days (Mennella and Moltz 1988). To the rat owner, it looks like the female conceived about 10 days after giving birth, when in fact she conceived shortly after birth but simply had a longer pregnancy than normal.
After the postpartum estrus, the female rat does not come into heat again until after the weaning of her litter. This estrus is called the postweaning estrus and it occurs about 29 days after a female rat gives birth (Mennella and Moltz 1988).