the release (letdown) of milk and its ejection from the nipple in a positive feedback loop. It was previously thought that prolactin hormone, which is released by the anterior pituitary in response to the direct nerve stimulation of suckling, was responsible for creating the hormonal pathways necessary to sustain amenorrhea. Now, however, it seems that this relationship is one of correlation not causation as prolactin levels in the blood plasma are simply an indicator of suckling frequency. Suckling, and the subsequent release of prolactin, is not directly responsible for postpartum infecundity. Rather it is one mechanism that increases milk production, thereby increasing the metabolic cost of breastfeeding to mothers, which contributes to sustained infecundity. Suckling as proxy indicator of infecundity rather than a direct, hormonal causal factor is supported in studies contrasting the nursing intensity hypothesis, which says that more intense (prolonged, frequent) breastfeeding will result in a longer period of lactational amenorrhea, and the metabolic load model, which posits that maternal energy availability will be the main factor determining postpartum amenorrhea and the timing of the return of ovarian function. Lactation and energy availability Postpartum ovarian function and the return of fecundity depend heavily on maternal energy availability. This is due to the relatively consistent metabolic costs of milk production across populations, which fluctuate slightly but represent a significant cost to the mother. The metabolic load hypothesis states that women with more available energy or caloric/metabolic resources will likely resume ovarian function sooner, because breastfeeding represents a proportionally lower burden on their overall metabolic function. Women with less available energy experience a proportionally higher burden due to breastfeeding and therefore have less surplus metabolic energy to invest in continued reproduction. The metabolic load model is therefore consistent with the nursing intensity hypothesis, in that more intense nursing increases the relative metabolic burden of breastfeeding on the mother. It also takes into account the overall energy supply of the mother in determining whether she has enough caloric/metabolic resources available to her to make reproduction possible. If net energy supply is high enough, a woman will resume ovarian cycling sooner despite still breastfeeding the current infant. Infecundity Amenorrhea itself is not necessarily an indicator of infecundity, as the return of ovarian cycling is a gradual process and full fecundity may occur before or after first postpartum menses. Additionally, spotting or the appearance of first postpartum menses can be a result of