JollyJunquirkPrtyThe Hormone Harmony Guide

Menstrual cycle & hormonal balance

How the menstrual cycle works — and why yours may differ

A plain-language guide to cycle phases, hormone changes, common symptoms and variability. Educational only: it cannot predict ovulation, fertility, pregnancy or diagnose a condition.

Published · Latest verified educational update · Sources listed below

Delicate pink flowers on soft stems, a decorative botanical image for cycle education

Cycle phases

Explore the four phases

Select a phase. Day ranges are examples from a commonly described cycle; real timing varies from person to person and cycle to cycle.

Menstruation (the period)

If pregnancy hasn't occurred, falling progesterone and estrogen cause the uterine lining to shed. Day 1 of bleeding is counted as day 1 of the cycle. Bleeding commonly lasts from about 2 to 7 days.

Commonly reported: cramping (often linked to prostaglandins), lower back ache, tiredness. Pain that stops you from doing usual activities is worth discussing with a clinician.

Hormone pathways

How hormones rise and fall

The hypothalamus releases GnRH, which signals the pituitary to release FSH and LH. These act on the ovaries, which produce estrogen and progesterone. The hormones feed back to the brain — a loop known as the hypothalamic–pituitary–ovarian (HPO) axis.

Stress, significant weight change, intense exercise, illness, thyroid conditions, PCOS, some medicines and hormonal contraception can all affect this loop.

✓ Established physiology
Illustrative hormone levels across one cycle Estrogen (solid line) rises before ovulation and again modestly in the luteal phase. LH (dotted line) spikes sharply around ovulation. Progesterone (dashed line) rises after ovulation and falls before the next period. Not to scale. Ovulation Period starts Next period
  • Estrogen — solid line
  • LH — dotted line
  • Progesterone — dashed line
Illustrative and not to scale. Hormone tests should be interpreted by a clinician.

Cycle variability & tracking

A “normal” cycle is a range, not a number. Adult cycles commonly fall somewhere around 21 to 35 days, and international gynaecology definitions (FIGO) describe normal frequency as 24 to 38 days. Cycles are often less regular in the first years after periods begin and during perimenopause.

What tracking can be useful for

  • Recording the first day of each period and how long bleeding lasts
  • Noting flow (for example, how often you change products) and clots larger than about a 2.5 cm coin
  • Logging pain, mood and other symptoms alongside dates
  • Bringing a few months of notes to an appointment

Privacy: this website does not collect or store cycle or symptom data. If you use a tracking app, review how it stores and shares reproductive health information.

Common reasons cycles change

  • Pregnancy or breastfeeding
  • Hormonal contraception or other medicines
  • Stress, illness, travel or disrupted sleep
  • Significant weight change or very intense exercise
  • Polycystic ovary syndrome (PCOS)
  • Thyroid disorders or raised prolactin
  • Fibroids, polyps or endometriosis
  • Perimenopause

Changes can have many causes. A change in your pattern is a reason to observe and, where it persists, to seek assessment — not proof of any one condition.

When professional assessment may be appropriate

◆ Book a routine appointment

  • Periods that stop for 3 months or more (and you're not pregnant)
  • Cycles regularly shorter than 21 or longer than 35 days
  • Bleeding between periods or after sex
  • Periods lasting longer than 7 days
  • Period pain or PMS that disrupts daily life
  • No period by age 15

▲ Seek urgent medical care

  • Soaking through a pad or tampon every hour for several hours
  • Sudden, severe pelvic or abdominal pain
  • Feeling faint, dizzy or very short of breath with bleeding
  • Sudden high fever, rash or feeling very unwell while using a tampon or menstrual cup
  • Bleeding or pain with a positive pregnancy test

In an emergency, call your local emergency number (for example 999 in the UK, 911 in the US, 112 in the EU).

Sources & further reading

Review dates for each source appear on the source's own page. Related guides: Menstrual cycle guide · PCOS · Thyroid