Lactational amenorrhea, also called postpartum infertility, is the temporary postnatal infertility that occurs when a woman is amenorrheic (not menstruating) and fully breastfeeding. Physiology Hormonal pathways and neuroendocrine control Breastfeeding delays the resumption of normal ovarian cycles by disrupting the pattern of pulsatile release of gonadotropin-releasing hormone (GnRH) from the hypothalamus and hence luteinizing hormone (LH) from the pituitary. The plasma concentrations of follicle-stimulating hormone (FSH) during lactation are sufficient to induce follicle growth, but the inadequate pulsatile LH signal results in a reduced estradiol production by these follicles. When follicle growth and estradiol secretion does increase to normal, lactation prevents the generation of a normal preovulatory LH surge and follicles either fail to rupture, or become atretic or cystic. Only when lactation declines sufficiently to allow generation of a normal preovulatory LH surge to occur will ovulation take place with the formation of a corpus luteum of variable normality. Thus lactation delays the resumption of normal ovarian cyclicity by disrupting but not totally inhibiting, the normal pattern of release of GnRH by the hypothalamus. The mechanism of disruption of GnRH release remains unknown. In women, hyperprolactinemia is often associated with amenorrhea, a condition that resembles the physiological situation during lactation (lactational amenorrhea). Mechanical detection of suckling increases prolactin levels in the body to increase milk synthesis. Excess prolactin may inhibit the menstrual cycle directly, by a suppressive effect on the ovary, or indirectly, by decreasing the release of GnRH. Suckling stimulus Suckling intensity directly correlates with the duration of the amenorrheal period following birth. Suckling intensity has several dynamic components: frequency of suckling, duration of the suckling bout, and duration of suckling in a 24 hour period. It is not clear which of these plays the most critical role in maintaining amenorrhea. Suckling intensity is highly variable across populations. Studies of U.S. and Scottish women show that at least six bouts per day and 60 minutes of suckling in a 24 hour period will typically sustain amenorrhea. Concurrent studies of !Kung women in Botswana and Gainj women in Papua New Guinea have shown that very frequent, very short suckling bouts of about 3 minutes, 40 to 50 times per day correlate with typical amenorrhea of up to two years postpartum. 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