During lactation, the hormone prolactin is increased due to the stimulation of the breast by the baby sucking on the nipple.
This high level of prolactin, in addition to stimulating milk production, inhibits the hypothalamus and prevents the secretion of GnRH (gonadotropin-releasing hormone). As a result, the entire hypothalamus-pituitary-ovarian axis is blocked and the woman will not have ovulation or menstruation until she finishes exclusive breastfeeding.