What Is the Menstrual Cycle: All Four Phases Explained by Science

What Is the Menstrual Cycle?

The menstrual cycle is a recurring hormonal process that prepares the female reproductive system for potential pregnancy. In the absence of fertilization, the cycle ends with menstruation — the shedding of the uterine lining — and resets. The average cycle length is 28 days, but normal ranges from 21 to 35 days are common, and cycles can vary from month to month in the same individual without indicating any disorder.

The cycle is driven by a tightly choreographed sequence of hormones from three locations: the hypothalamus (releasing GnRH), the pituitary gland (releasing FSH and LH), and the ovaries (producing estrogen and progesterone). These hormones act on the ovaries and uterus simultaneously, preparing an egg for release while building the uterine lining to receive a fertilized embryo. Understanding all four phases reveals far more than reproductive biology — these hormones affect mood, cognition, energy, skin, and physical performance throughout the cycle.

Phase 1: Menstrual Phase (Days 1-5)

The cycle officially begins on day 1 of menstruation — the first day of actual bleeding. When a previous cycle's egg goes unfertilized, progesterone and estrogen levels fall sharply. This drop signals the uterus to shed its thickened lining (the endometrium) through the cervix and vagina, producing menstrual flow that lasts 3 to 7 days on average.

The fall in progesterone triggers the release of prostaglandins, hormone-like compounds that cause uterine muscle contractions to expel the lining. Prostaglandins are responsible for menstrual cramps (dysmenorrhea); their intensity varies widely between individuals and across different cycles. NSAIDs like ibuprofen work by blocking prostaglandin synthesis, which is why they are more effective taken before cramping begins than after.

Phase 2: Follicular Phase (Days 1-13)

The follicular phase overlaps with menstruation, beginning on day 1 and extending to ovulation. During this phase, the pituitary gland releases follicle-stimulating hormone (FSH), which stimulates several ovarian follicles to develop. Each follicle contains an egg (oocyte). Typically, one follicle becomes dominant and the others are reabsorbed — a process called follicle selection.

As the dominant follicle grows, it produces increasing amounts of estrogen, which causes the uterine lining to thicken and proliferate in preparation for a potential implantation. Rising estrogen levels produce several noticeable effects beyond the reproductive tract:

• Energy and mood often improve as estrogen rises, peaking in the late follicular phase. Many women report feeling most energetic, socially engaged, and cognitively sharp in the days before ovulation.
• Skin appearance often improves; estrogen promotes collagen synthesis and sebum regulation.
• Verbal fluency and fine motor skills show modest improvements in research on hormone-cognition relationships during the estrogen-dominant follicular phase.

Phase 3: Ovulation (Day 14, approximately)

Ovulation occurs when a surge of luteinizing hormone (LH) — triggered by estrogen reaching a peak concentration — causes the dominant follicle to rupture and release the mature egg. The egg travels down the fallopian tube toward the uterus. It remains viable for fertilization for 12 to 24 hours, while sperm can survive in the female reproductive tract for up to 5 days, creating a fertile window of approximately 5 to 6 days centered on ovulation.

The LH surge typically occurs 24 to 36 hours before ovulation and is detectable by urine-based ovulation predictor kits (OPKs), which are commonly used for both fertility planning and cycle tracking. Physical symptoms at ovulation can include a sharp, one-sided pain called mittelschmerz (German for middle pain), increased cervical mucus with an egg-white texture, a slight rise in basal body temperature, and for some women, a brief increase in libido.

Phase 4: Luteal Phase (Days 15-28)

After ovulation, the ruptured follicle transforms into the corpus luteum — a temporary endocrine structure that produces progesterone as its primary hormone, with estrogen continuing at lower levels. Progesterone prepares the uterine lining for potential implantation, making it thicker, more vascular, and receptive to an embryo. Body temperature rises approximately 0.2 to 0.5 degrees Celsius after ovulation and remains elevated throughout the luteal phase — a pattern used in fertility awareness methods.

If fertilization does not occur, the corpus luteum degenerates after approximately 14 days, causing progesterone and estrogen to fall. This hormonal withdrawal triggers menstruation, beginning a new cycle. The progesterone-dominant luteal phase produces distinct physiological and psychological changes:

• Premenstrual syndrome (PMS): The drop in progesterone in the late luteal phase causes the constellation of physical and emotional symptoms — bloating, breast tenderness, irritability, mood shifts — experienced by up to 75 percent of cycling women to some degree.
• PMDD: Premenstrual Dysphoric Disorder is a severe form of PMS marked by significant depression, anxiety, or irritability severe enough to impair daily functioning. It affects 3 to 8 percent of cycling individuals and responds to SSRIs and, in some cases, hormonal management.
• Sleep quality can worsen as progesterone initially promotes drowsiness but fragments sleep in the late luteal phase.

Hormonal Influences on Performance and Cognition

Research on how cycle phases affect exercise performance, pain sensitivity, and cognitive function has grown substantially in recent years. Estrogen appears to have a mild protective effect on muscle and connective tissue; some research suggests injury risk may be slightly higher in the late follicular phase. During the luteal phase, glycogen utilization and oxygen consumption during exercise change modestly due to progesterone.

Tracking the menstrual cycle — using apps, basal body temperature monitoring, or cervical mucus observation — provides insight into reproductive health beyond fertility. Irregular cycles, extremely heavy periods, severe PMS, or cycles shorter than 21 or longer than 35 days can indicate underlying conditions including thyroid disorders, PCOS, endometriosis, or hypothalamic amenorrhea that benefit from medical evaluation.