When an infant suckles, sensory receptors in the nipple send a signal to the anterior pituitary gland in the brain, which secretes prolactin and oxytocin. Prolactin and oxytocin trigger 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.