Resumption of fertility following parturition is dependent to variable degrees in different species on two main factors - intensity of lactation and recovery of the uterus to its nonpregnant status. Lactation has a strong suppressive influence on cyclicity in some species (eg., human). In other mammals lactation has either a weak (eg., dairy cattle) or no appreciable (eg., rodents, horse) affect on return to cyclicity after parturition and uterine involution is the rate-limiting event governing postpartum fertility (Table 5-16). The puerperium is the period from delivery until involution of the uterus is complete (eg., in women this takes ~ 45 days).
Mechanisms by which suckling and the metabolic demand of lactation inhibit fertility are not entirely clear. The normal cyclic discharge of gonadotropins is generally disrupted during lactation. In women the inhibitory effect of lactation on menstrual cyclicity involves prolactin; drugs that antagonize secretion of prolactin will reinstate menstrual cycles (although early weaning of calves shortens the postpartum interval of beef cows, bromocryptine is not correspondingly effective).