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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The endometrium sheds because the corpus luteum from the previous cycle has involuted and progesterone has withdrawn.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.

Frequently asked questions

What is a normal menstrual cycle length?
Current FIGO criteria define a normal cycle as 24 to 38 days, measured from the first day of one period to the first day of the next. The classic "28-day cycle" is an average, not a requirement, and cycle-to-cycle variation of up to about a week is still considered normal. Cycles that are consistently shorter than 24 days, longer than 38 days, or that swing widely are worth evaluating.
How can I tell whether and when I ovulate?
Ovulation usually occurs about 24 to 36 hours after the mid-cycle LH surge, roughly 14 days before the next period, because the luteal phase is relatively fixed at about 14 days. Practical signs include clearer, stretchier cervical mucus and sometimes mid-cycle (mittelschmerz) pain. The simplest reliable confirmation that ovulation has occurred is a mid-luteal serum progesterone (around day 21 of a 28-day cycle).
What are the common causes of irregular periods?
Irregular cycles usually reflect a disturbance somewhere along the hypothalamic-pituitary-ovarian axis. Common causes include polycystic ovary syndrome (PCOS), thyroid disease, significant weight gain or loss, intense exercise or psychological stress, a raised prolactin level, and, in older women, the approach of menopause. Evaluation works methodically down this axis.
When should I see a doctor about my menstrual cycle?
Reasonable reasons to seek assessment include cycles persistently shorter than 24 days or longer than 38 days, periods that stop for three months or more outside of pregnancy, very heavy or prolonged bleeding, bleeding between periods or after intercourse, or difficulty conceiving after 6 to 12 months of trying. These findings do not always indicate a serious problem, but they deserve proper evaluation.
Can I get pregnant at any point in my cycle?
Conception is only possible around ovulation. The fertile window spans roughly the five days before ovulation plus the day of ovulation itself, because sperm can survive several days in receptive cervical mucus while the egg is fertilisable for only about 12 to 24 hours. Because ovulation timing naturally varies, no day can be treated as entirely "safe", and cycle tracking gives an estimate rather than a guarantee.

Frequently asked questions

What is a normal menstrual cycle length?
Current FIGO criteria define a normal cycle as 24 to 38 days, measured from the first day of one period to the first day of the next. The classic "28-day cycle" is an average, not a requirement, and cycle-to-cycle variation of up to about a week is still considered normal. Cycles that are consistently shorter than 24 days, longer than 38 days, or that swing widely are worth evaluating.
How can I tell whether and when I ovulate?
Ovulation usually occurs about 24 to 36 hours after the mid-cycle LH surge, roughly 14 days before the next period, because the luteal phase is relatively fixed at about 14 days. Practical signs include clearer, stretchier cervical mucus and sometimes mid-cycle (mittelschmerz) pain. The simplest reliable confirmation that ovulation has occurred is a mid-luteal serum progesterone (around day 21 of a 28-day cycle).
What are the common causes of irregular periods?
Irregular cycles usually reflect a disturbance somewhere along the hypothalamic-pituitary-ovarian axis. Common causes include polycystic ovary syndrome (PCOS), thyroid disease, significant weight gain or loss, intense exercise or psychological stress, a raised prolactin level, and, in older women, the approach of menopause. Evaluation works methodically down this axis to identify the level at which the signal is disrupted.
When should I see a doctor about my menstrual cycle?
Reasonable reasons to seek assessment include cycles persistently shorter than 24 days or longer than 38 days, periods that stop for three months or more outside of pregnancy, very heavy or prolonged bleeding, bleeding between periods or after intercourse, or difficulty conceiving after 6 to 12 months of trying. These findings do not always indicate a serious problem, but they deserve proper evaluation rather than reassurance alone.
Can I get pregnant at any point in my cycle?
Conception is only possible around ovulation. The fertile window spans roughly the five days before ovulation plus the day of ovulation itself, because sperm can survive several days in receptive cervical mucus while the egg is fertilisable for only about 12 to 24 hours. Because ovulation timing naturally varies, no day can be treated as entirely "safe", and cycle tracking gives an estimate rather than a guarantee.
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