The menstrual cycle explained

What the cycle is

The menstrual cycle is the regular pattern of hormonal and physical changes that prepares the body for potential pregnancy each month.

How long it lasts

A typical cycle runs from 21 to 35 days, measured from the first day of one period to the first day of the next. Most cycles are around 28 days but variation within this range is entirely normal. Cycles outside this range warrant attention. Cycle length varies between women and within one woman across her reproductive life.

When it starts and ends

Cycles begin at puberty (menarche) typically around age 12 to 13. They become more regular by age 16 to 17 for most women. Cycles continue through reproductive life until they end at menopause, on average at age 51 in the UK. The first 2 to 3 years and the years leading to menopause both involve more irregularity than the stable reproductive years in between.

Why it happens

The cycle exists to support potential pregnancy. Each month an egg matures and is released. The uterine lining thickens to receive a potential pregnancy. If no pregnancy occurs, the lining sheds as a period and the cycle starts again. This monthly preparation continues across reproductive years even if no pregnancy is desired or possible.

What controls it

The hypothalamus in the brain controls the cycle through hormonal signals to the pituitary gland, which signals the ovaries. The whole system runs as a feedback loop, with hormone levels rising and falling in characteristic patterns. The complexity of this system explains why so many things can affect the cycle.

What happens through the month

The cycle has four distinct phases, each with its own hormonal pattern and characteristic features.

The menstrual phase

The cycle starts on the first day of your period. The uterine lining built up over the previous cycle sheds. Oestrogen and progesterone are at their lowest. Most periods last 3 to 7 days. Total blood loss is typically 30 to 80 ml. Many women feel tired in the first day or two then increasingly themselves as the period progresses.

The follicular phase

From the end of your period to ovulation. Eggs develop in the ovaries. Oestrogen rises steadily. The uterine lining rebuilds. This phase is the most variable in length. Women with longer cycles usually have longer follicular phases. Most women feel energetic, focused and stable through this phase as oestrogen does its work.

Ovulation

A brief midpoint when an egg is released. A surge of LH triggers the release. The egg can be fertilised over the next 12 to 24 hours. Cervical mucus becomes clear and stretchy. Some women feel a slight pain on one side or notice other subtle changes. Ovulation typically happens 12 to 14 days before the next period rather than 14 days after the previous one.

The luteal phase

From ovulation to the next period. The empty follicle becomes the corpus luteum and produces progesterone. The uterine lining prepares for potential pregnancy. Body temperature rises slightly. PMS happens for women who have it, typically in the week before the period. If no pregnancy occurs, hormones fall and the next period starts.

Understanding variation

Wide variation in cycles is normal. Knowing what counts as normal helps identify what does not.

Cycle length variation

Cycles between 21 and 35 days are within the normal range. Cycle to cycle variation of 7 to 9 days within the same woman is normal. Significant variation outside this range warrants attention. Cycles get more irregular at the beginning of reproductive life (adolescence) and at the end (perimenopause). The middle decades typically have the most consistent cycles.

Period variation

Period length, flow and symptoms vary between women and across cycles within the same woman. Most periods last 3 to 7 days. Heavier flow in the first 1 to 2 days is typical. Some clotting is normal. Cramps in the first 1 to 2 days are common. Significant variation between your own cycles can be normal in adolescence and perimenopause.

Symptom variation

Most women notice some cyclical changes including breast tenderness, mood changes, energy variations, bloating and others. The pattern is typically consistent for each woman across cycles. Significant changes from your normal pattern warrant attention. Knowing your usual experience helps you spot anything unusual.

When variation is concerning

Cycles consistently shorter than 21 days or longer than 35 days, periods heavy enough to affect daily life, severe pain that interferes with normal activities, bleeding between periods, sudden significant changes in your usual pattern, missed periods (other than pregnancy) and bleeding after menopause all warrant medical attention. Speak to your GP.

How the cycle changes

The cycle pattern shifts across reproductive life. Knowing what each stage typically brings helps you make sense of your own experience.

Early years

The first 2 to 3 years of cycles are often irregular as the hormonal system matures. Ovulation may not happen in every cycle. Cycles gradually become more regular through the late teens. Heavy periods and severe pain in adolescence are not normal and warrant medical attention even though they are common. Speak to a GP.

Reproductive years

Cycles are generally most regular and predictable through the twenties and into the thirties. PMS can become more prominent through these decades for some women. Pregnancy and breastfeeding pause cycles. Hormonal contraception affects what cycles look like, in many cases producing more predictable bleeding patterns than natural cycles would.

Approaching menopause

Cycles start changing in perimenopause, typically beginning in the early to mid forties. Cycles often shorten initially then become more irregular over time. Heavier or different periods are common. Eventually periods are skipped and become less predictable. Many other perimenopausal symptoms appear during these years. Speak to your GP if symptoms are affecting your life.

Menopause and beyond

Menopause is reached after 12 consecutive months without a period. Cycles end. The reproductive system has effectively retired. Postmenopausal bleeding always warrants prompt medical assessment because it has many possible causes including some that need treatment.

More on female health

For the phase by phase breakdown, our The Four Hormonal Phases of the Menstrual Cycle covers each phase in detail. Common Menstrual Irregularities and What Causes Them covers when cycles go off pattern. And The Key Hormones That Drive Female Health covers the hormones running the show.