Menstrual Cycle Phase Calculator

The menstrual cycle is divided into four phases, each driven by a distinct hormonal pattern. The classic 28-day cycle is a statistical average — cycles from 21 to 35 days are considered normal, and the length often varies from month to month.

Phase 1 — Menstruation (days 1 to period duration): Progesterone and estrogen drop when pregnancy does not occur, triggering the shedding of the uterine lining. FSH begins rising near the end of this phase.

Phase 2 — Follicular phase (period end to ovulation-2): FSH stimulates ovarian follicles to develop and produce estrogen. Estrogen rises steadily, thickening the uterine lining. Energy tends to peak in this phase. The follicular phase is the most variable part of the cycle — short cycles compress it, long cycles extend it.

Phase 3 — Ovulation (approximately cycle day = cycle length minus 14): A surge in LH (luteinizing hormone) triggers the release of the dominant follicle’s egg. This phase lasts 24-48 hours. Basal body temperature rises slightly after ovulation, which is why BBT charting helps confirm (but not predict) ovulation after the fact.

Phase 4 — Luteal phase (ovulation+1 to cycle end): The empty follicle becomes the corpus luteum and produces progesterone. This phase is relatively constant at 12-16 days regardless of cycle length. If fertilization occurs, HCG from the embryo maintains the corpus luteum; if not, progesterone drops and the cycle restarts.

Ovulation is estimated as day (cycle length - 14). This is an average; the fertile window spans roughly five days before ovulation through the day of ovulation itself.