The four phases of the cycle

When we say “cycle”, most of us picture the days of bleeding. But they’re only the beginning. Over the rest of the month your body does quiet, precise work – and it explains a lot of what you feel from week to week.

How long a cycle lasts

The cycle is counted from the first day of your period up to the first day of your next one.⁵ It is regulated by hormones and, according to the World Health Organization, lasts on average between 21 and 35 days.¹ The US Office on Women’s Health gives slightly different limits: your periods are still “regular” if they usually come every 24 to 38 days.⁵ The difference shows something important: “normal” is a wide range.

Phase 1: Menstruation

The first day of bleeding is day one of the cycle. If there is no pregnancy, levels of the hormones oestrogen and progesterone fall. Very low levels tell your body to begin menstruation.⁵ The lining of the womb comes away and leaves the body.³

Bleeding lasts between 2 and 7 days, usually about 5. You typically lose about 20 to 90 ml of blood – roughly 1 to 5 tablespoons – although some women bleed more heavily.² Pain is common too: according to the WHO, more than 2 out of 3 women and girls have pain with menstrual bleeding.¹

Phase 2: The follicular phase

It starts together with your period, on day one, and ends at ovulation.⁴ Follicle-stimulating hormone (FSH) makes several follicles in the ovaries grow, each holding an egg. Oestrogen rises and the womb lining starts to thicken again.^3,4

Phase 3: Ovulation

In a 28-day cycle, ovulation happens around day 14. A sudden rise in luteinising hormone (LH) makes the ovary release its egg.⁴ In most women this happens 10 to 16 days before the next period.³

That’s why, in a shorter or longer cycle, ovulation isn’t on day 14. Sperm can survive for up to 7 days after sex,³ so the days when pregnancy is possible begin before ovulation itself.

Phase 4: The luteal phase

In a 28-day cycle it runs from about day 15 to day 28. The egg travels through the fallopian tube towards the womb, and progesterone rises to prepare the lining for a possible pregnancy.⁴ If there is no pregnancy, oestrogen and progesterone fall, the lining comes away – and the cycle starts again.³

In the weeks before a period, many women notice premenstrual symptoms – mood changes, bloating, breast tenderness, tiredness, trouble sleeping.⁶

The cycle isn’t just the days you bleed. It’s the whole month.

The cycle changes over the years

• Periods usually begin at around the age of 12.² For a few years after the first one, cycles longer than 38 days are common. They usually become more regular within three years.⁵
• Periods are often heavier in your teens and usually get lighter in your 20s and 30s.⁵
• In your 40s, as your body starts the transition to menopause, cycles may become irregular.⁵ Menopause usually happens between your mid-40s and mid-50s.²

Why it’s worth tracking your cycle

You don’t need an app – a notebook will do. The US Office on Women’s Health suggests noting down:⁵

1. when your bleeding begins;
2. how many days it lasts;
3. how heavy it is;
4. which symptoms you have during your period;
5. which premenstrual symptoms you have.

After a few months you’ll see your own rhythm. And if something changes, you’ll have something to take to a doctor.

When to see a doctor

• if your cycles are longer than 38 days or shorter than 24;⁵
• if you bleed between periods, after sex or after the menopause;²
• if you’ve missed 3 periods in a row and a pregnancy test is negative;²
• if you need to change your pad or tampon every 1–2 hours, your period lasts more than 7 days or you bleed through to your clothes or bedding.⁷

The WHO puts it plainly: concerns about menstruation deserve to be heard and evaluated, because they can greatly reduce quality of life.¹

Sources

1. World Health Organization. Menstrual health – fact sheet, 18 June 2026.
2. NHS (UK National Health Service). Periods.
3. NHS. Fertility in the menstrual cycle.
4. Cleveland Clinic. Menstrual Cycle (Normal Menstruation).
5. Office on Women’s Health, US Department of Health and Human Services. Your menstrual cycle, 13 January 2025.
6. NHS. PMS (premenstrual syndrome).
7. NHS. Heavy periods.
8. European Parliamentary Research Service (EPRS). Addressing menstrual poverty in the EU, May 2025.