The menstrual cycle is a recurring, tightly coordinated series of physiologic changes in the female reproductive system, averaging 28 days with a normal range of 21–35 days. It is governed by the hypothalamic-pituitary-ovarian (HPO) axis and unfolds as two overlapping cycles that occur simultaneously: the ovarian cycle (follicular phase → ovulation → luteal phase) and the uterine (endometrial) cycle (menstrual → proliferative → secretory). These two cycles are synchronized by ovarian hormones, so the same days that mark follicular growth in the ovary also mark proliferation of the uterine lining, and the luteal phase corresponds to the secretory endometrium that prepares for implantation. A reliable mental model for nurses is that the cycle begins on the first day of menstrual bleeding (cycle day 1) and resets when the next period begins. Rising and falling levels of estrogen and progesterone drive every observable change a client may report — menstrual flow, mid-cycle mucus changes, breast tenderness, mood shifts, and basal body temperature patterns. Understanding this rhythm is foundational across nearly every reproductive-health encounter: dating a pregnancy by last menstrual period, counseling...