Anestrus
Anestrus refers to the phase when the sexual cycle rests. This is typically a seasonal event and controlled by light exposure through the pineal gland that releases melatonin. Melatonin may repress stimulation of reproduction in long-day breeders and stimulate reproduction in short-day breeders. Melatonin is thought to act by regulating hypothalamic pulse activity of gonadotropin-releasing hormone. Anestrus is induced by time of year, pregnancy, lactation, significant illness, chronic energy deficit, and possibly age.
After completion (or abortion) of a pregnancy, some species have postpartum estrus, which is ovulation and corpus luteum production that occurs immediately following the birth of the young. For example, the mouse has a fertile postpartum estrus that occurs 14–24 hours following parturition.
Read more about this topic: Estrous Cycle