How long is a normal menstrual cycle, and what changes it?

The 28-day cycle is an average, not a rule. Only a minority of people have cycles of exactly 28 days, and a cycle a few days longer or shorter than last month is completely ordinary. This guide sets out the normal range, explains which part of the cycle changes length, and lists the patterns that deserve a medical opinion. If you want your own numbers rather than averages, the period tracker works them out from your dates.

The normal range

Cycle length is counted from the first day of one period to the first day of the next. For adults, cycles of roughly 21 to 35 days are generally considered normal, and the NHS describes an even wider range of 21 to 40 days. Periods themselves usually last two to seven days, with the heaviest bleeding in the first two. Variation from month to month is normal too: many people's cycles shift by a few days, and occasionally by a week, without anything being wrong. What matters more than any single number is your own pattern and whether it changes.

The four phases

The cycle is driven by hormones from the brain and ovaries and is usually described in four phases:

• Menstruation — the lining of the uterus is shed; this is the period, and day 1 of the cycle.
• Follicular phase — from the start of the period until ovulation, follicles in an ovary mature and oestrogen rises, rebuilding the lining.
• Ovulation — a surge of luteinising hormone (LH) releases an egg, which survives about 12 to 24 hours.
• Luteal phase — the empty follicle becomes the corpus luteum and produces progesterone; if no pregnancy occurs, hormone levels fall and the next period begins.

The key fact for predictions is that the luteal phase is fairly steady, typically about 12 to 16 days. Most of the difference between a 25-day and a 35-day cycle comes from the follicular phase, which is why ovulation is estimated by counting back from the next period rather than forward from the last one, as the ovulation calculator does. The fertile window: which days you can get pregnant goes into that.

Why cycles vary

Because ovulation can be delayed, anything that affects the hormones before ovulation can lengthen a cycle. Common reasons include stress, illness, travel and jet lag, big changes in weight, intense training or low energy intake, and breastfeeding. Hormonal contraception changes the pattern entirely: the bleed on the pill is a withdrawal bleed, not a natural period. Medical causes of irregular cycles include polycystic ovary syndrome (PCOS), thyroid disorders and raised prolactin. Pregnancy is the most common reason for a missed period, so a late period with any chance of pregnancy is worth a test.

Cycles across a lifetime

For the first year or two after periods begin, cycles are often irregular and can be longer, because ovulation is not yet happening every month. Through the twenties and thirties most people settle into a fairly consistent pattern. In the years before menopause (perimenopause, often from the mid-forties) cycles commonly become shorter at first and then more irregular, with skipped periods, before stopping. Menopause is defined as 12 months without a period, on average around age 51.

When to see a doctor

Variation is normal, but these patterns are worth discussing with a doctor or nurse:

• cycles regularly shorter than 21 days or longer than 35, or a sudden change in your usual pattern;
• no period for three months (when not pregnant, breastfeeding or on contraception that stops periods);
• periods lasting more than seven days, very heavy bleeding, or passing large clots;
• bleeding between periods, after sex or after menopause;
• pain severe enough to stop normal activities.

Bring dates if you can; a few months of records make the conversation much faster. How to track your period: what to log and what it tells you covers what to write down, and if you are tracking symptoms like mood alongside your cycle, the mood journal keeps them in the same habit.