Human Reproduction: The Menstrual Cycle

Chemical Coordination and Integration already covers the pituitary gland's own general structure and hormone output, including FSH and LH among the anterior pituitary's hormones. This note covers how those same two hormones, together with the ovary's own estrogen and progesterone, actually drive the human menstrual cycle month to month.

The three phases of the ovarian cycle

The human menstrual cycle runs through three named phases, each dominated by a different hormone relationship between the pituitary and the ovary.

The follicular phase begins on the first day of menstrual bleeding and runs until ovulation. Through this phase, follicle-stimulating hormone (FSH) drives the growth and maturation of an ovarian follicle, and as that follicle develops, it secretes rising levels of estrogen. This is the phase's own genuinely important mechanical detail: rising estrogen does not simply keep building steadily, it eventually triggers a sharp, sudden surge in luteinising hormone (LH) from the pituitary, a positive-feedback trigger point rather than a gradual response, and this LH surge is specifically what makes ovulation happen, causing the mature egg to actually rupture out of its follicle and be released from the ovary.

Ovulation itself is the brief middle event, the egg's release, sitting at the hinge point between the two longer phases on either side of it.

The luteal phase, which follows ovulation, is considerably more consistent in length across individuals than the follicular phase (typically close to 14 days), and it is dominated by a structure the ruptured follicle itself transforms into, the corpus luteum, which is in turn stimulated by LH to secrete large amounts of progesterone, the dominant hormone of this phase. Progesterone's own job is to prepare and maintain the uterine lining for a potential pregnancy; if fertilisation does not occur, the corpus luteum breaks down, progesterone levels fall, the uterine lining is shed as menstrual bleeding, and a new cycle's follicular phase begins again.

Quick revision points

• Menstrual cycle, three phases: follicular (begins at menstruation, FSH drives follicle growth, follicle secretes rising estrogen), ovulation (egg release), luteal (fairly consistent ~14-day length, dominated by progesterone from the corpus luteum).
• The LH surge, triggered by rising estrogen from the developing follicle, is specifically what causes ovulation, a positive-feedback trigger point rather than a gradual hormonal drift.
• Luteal phase: the ruptured follicle becomes the corpus luteum, stimulated by LH to secrete progesterone, which maintains the uterine lining; if fertilisation does not occur, the corpus luteum degrades, progesterone falls, and menstruation begins the next cycle.