Educational reference · Reproductive physiology

Menstrual cycle physiology — hormones, phases, and what variation means

The menstrual cycle is the monthly preparation of the reproductive system for a possible pregnancy. It is governed by a tightly regulated conversation between the brain and the ovaries — the hypothalamic–pituitary–ovarian (HPO) axis. This page describes how the cycle works, what counts as normal variation, and when irregularity should prompt evaluation.

The HPO axis at a glance

• The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses.
• GnRH signals the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinising hormone (LH).
• FSH and LH act on the ovaries to stimulate follicle growth, ovulation, and corpus luteum formation.
• The ovaries produce oestradiol and progesterone, which in turn feed back to the hypothalamus and pituitary — sometimes inhibiting them, sometimes stimulating them — to drive the next phase of the cycle.

Disturbance at any level of this axis (stress, weight extremes, thyroid disease, PCOS, hypothalamic amenorrhoea, prolactinoma, premature ovarian insufficiency) can disrupt the cycle. Diagnostic evaluation, when needed, walks down this axis level by level.

The four functional phases

1. Menstrual phase (days 1 to ~5)

The endometrium sheds because the corpus luteum from the previous cycle has involuted and progesterone has withdrawn. Menstrual flow is typically 30 to 80 mL over 3 to 7 days. Day 1 of the cycle, by convention, is the first day of full menstrual flow (not spotting).

2. Follicular phase (days ~1 to 13)

Rising FSH recruits a cohort of antral follicles. Through a process of selection, one dominant follicle emerges by approximately day 7 to 8, while the others undergo atresia. The dominant follicle produces increasing oestradiol, which thickens the endometrium (the proliferative endometrium) and prepares the cervical mucus to support sperm transport.

3. Ovulatory phase (days ~13 to 15)

When oestradiol reaches a critical threshold and is sustained for approximately 48 hours, the negative feedback on the pituitary switches to positive feedback — triggering a sharp LH surge. Ovulation follows the LH surge by approximately 24 to 36 hours. Some patients experience mittelschmerz (mid-cycle pain) and may notice clearer, stretchier cervical mucus.

4. Luteal phase (days ~15 to 28)

The ruptured follicle becomes the corpus luteum, which secretes progesterone. Progesterone transforms the endometrium into a secretory state suitable for implantation. The luteal phase is relatively fixed in length at approximately 14 days. If conception occurs, the corpus luteum is maintained by human chorionic gonadotropin (hCG) from the developing embryo; if not, it involutes and the cycle restarts.

Hormone dynamics through the cycle

• FSH rises in the early follicular phase, then falls as oestradiol rises and feeds back negatively.
• Oestradiol rises steadily through the follicular phase, peaks just before ovulation, dips briefly, then rises again in the luteal phase before falling at the end of the cycle.
• LH remains low through most of the cycle except for the sharp mid-cycle surge that triggers ovulation.
• Progesterone is low in the follicular phase and rises sharply in the luteal phase after ovulation — which is why a mid-luteal serum progesterone is a useful test of whether ovulation has occurred.

What counts as a normal cycle

• Cycle length — 24 to 38 days, per current FIGO definitions. The classic “28-day cycle” is an average rather than a rule.
• Duration of flow — 3 to 8 days
• Volume of flow — broadly normal if there are no large clots, no soaking through pad or tampon every 1 to 2 hours, and no flooding
• Variation — cycle-to-cycle variation up to 7 to 9 days is within normal limits in healthy adults

Adolescent cycles in the first 2 to 3 years after menarche are often anovulatory and irregular. Perimenopausal cycles also become more variable. Both are physiological.

Signs that ovulation has occurred

• A sustained rise in basal body temperature of approximately 0.3 to 0.5 °C in the luteal phase
• Mid-cycle change in cervical mucus from sticky to clear and stretchy
• Mid-cycle pain or spotting in some patients
• Positive urinary LH test (commercial ovulation predictor kits) approximately 24 to 36 hours before ovulation
• A mid-luteal serum progesterone above the laboratory threshold for ovulation

No single sign is universally reliable. For couples timing intercourse, the fertile window is approximately the 6 days ending on the day of ovulation, with the highest probability on the day before and the day of ovulation.

When the cycle warrants evaluation

Please consult a gynaecologist if any of the following apply:

• Cycles consistently shorter than 24 days or longer than 38 days
• Missed periods for 3 or more months in a row, outside pregnancy or hormonal contraception
• Heavy menstrual bleeding — flooding, large clots, anaemia, or interference with daily life
• Inter-menstrual bleeding or post-coital bleeding
• New or worsening dysmenorrhoea (period pain) that limits function or does not respond to simple analgesia
• Suspected oligomenorrhoea (infrequent periods) or amenorrhoea (absent periods) after previously regular cycles
• Difficulty conceiving after 12 months of unprotected intercourse (or 6 months if the patient is over 35)

Initial evaluation is typically clinical and includes targeted blood tests (TSH, prolactin, AMH, day-2 to day-3 FSH and oestradiol depending on context) and pelvic ultrasound. Further work-up depends on findings.

Cycle tracking — useful, but with limits

Tracking apps and wearables are helpful for recording menstrual data and identifying patterns. They are less reliable when used as standalone contraception, particularly in cycles that are short, long, or irregular. The fertile window in such cycles is harder to predict, and modelled estimates may not match true ovulation. For contraception, a method validated for that purpose is preferred. For symptom tracking, period and symptom logs are a useful adjunct to a clinical consultation.

Related reading

For underlying anatomy, see female reproductive anatomy. For conditions where the cycle is the lead symptom, see PCOS, endometriosis, and general gynaecology. For fertility evaluation, see IVF and fertility.

Dr. Priyadatt Patel

Senior Gynecologist · Advanced Laparoscopic Surgeon · IVF and Endometriosis Programme Lead

MS OBGyn · Pregnancy Care · Advanced Gynaecological Ultrasound · Fertility Preservation

ESHRE / ESGE / AAGL / ASRM guideline-aligned practice. 3D Karl Storz precision technique. Fertility-preservation-first philosophy. Evidence-based decisions, honest counselling, long-term outcomes orientation.