Why can menstruation be absent during lactation?

During lactation, the absence of menstruation—a state known as amenorrhea—is primarily driven by the hormonal changes required to sustain milk production.1

When a mother breast-feeds, the suckling infant stimulates the pituitary gland to release high concentrations of the hormone prolactin. These elevated levels of prolactin inhibit the secretion of gonadotropin-releasing hormone (GnRH) from the [hypothalamus].23

Because GnRH is necessary to trigger the release of gonadotropins—specifically luteinizing hormone and follicle-stimulating hormone—their levels decrease, which in turn suppresses cyclic ovarian function. Without the normal stimulation of the ovaries by these gonadotropins, the menstrual cycle is disrupted, leading to the temporary cessation of periods. This physiological mechanism serves to reduce fertility and protect the lactating woman from a premature pregnancy.31