Menstrual cycle

The menstrual cycle is the roughly monthly process where the body prepares for a possible pregnancy and then resets if one doesn't happen. It involves hormones rising and falling, an egg being released (ovulation), the uterus lining building up, and — if there's no pregnancy — that lining being shed as a period. The whole thing then starts again.

• The menstrual cycle is the recurring hormonal process that controls ovulation and periods.
• It's usually described as 28 days, but anything from 21 to 35 days is normal. Many people's cycles aren't perfectly regular.
• Day 1 is the first day of your period. Ovulation usually happens roughly mid-cycle.
• The cycle is driven by four main hormones: oestrogen, progesterone, FSH, and LH.
• Understanding your cycle helps you understand periods, fertility, mood changes, and how hormonal contraception works.

[DIAGRAM: Circular timeline diagram of the menstrual cycle showing hormone levels (oestrogen, progesterone, LH, FSH), uterine lining changes, and ovulation across ~28 days]

The four phases

The cycle is usually broken into four phases, though they flow into each other:

1. Menstruation (the period) — roughly days 1-5

The uterus lining sheds because no pregnancy happened in the previous cycle. This is the bleeding part. It lasts anywhere from 2 to 7 days. Day 1 of your period is considered day 1 of your cycle.

2. The follicular phase — roughly days 1-13

This overlaps with menstruation. The brain releases FSH (follicle-stimulating hormone), which tells the ovaries to start maturing a batch of eggs. Usually one egg becomes the dominant one. Meanwhile, oestrogen rises, telling the uterus lining to start building up again — getting thick and blood-rich in case a fertilised egg needs to implant.

3. Ovulation — roughly day 14

A surge of LH (luteinising hormone) triggers the release of the mature egg from the ovary. The egg travels into the fallopian tube. This is the fertile window — the time when pregnancy is possible. The egg survives for about 12-24 hours, but sperm can survive in the body for up to 5 days, so the window of potential fertility is about 6 days.

4. The luteal phase — roughly days 15-28

After the egg is released, the empty follicle turns into something called the corpus luteum, which produces progesterone. Progesterone maintains the uterus lining and tells the body to stay ready for pregnancy. If the egg isn't fertilised, the corpus luteum breaks down, progesterone drops, and the lining starts to shed — bringing you back to day 1.

Why "28 days" is misleading

The textbook cycle is 28 days, but in real life it varies a lot. Cycles of 21-35 days are all considered normal. Some people's cycles are like clockwork; others vary by several days each month. Stress, illness, travel, weight changes, and exercise can all shift the timing. During the first few years after periods start, irregular cycles are especially common.

Tracking your cycle — even just noting when your period starts each month — can help you spot your own pattern and notice if something changes.

What you might feel

The hormonal shifts during the cycle can affect more than just your uterus:

• During your period — cramps, tiredness, headaches, bloating. Some people feel fine; others have a rough few days.
• After your period (follicular phase) — oestrogen is rising. Many people feel more energetic and upbeat during this phase.
• Around ovulation — some people notice a higher sex drive, clearer skin, or a mild twinge of pain on one side (mittelschmerz). Cervical mucus becomes clear and stretchy.
• Before your period (luteal phase) — progesterone is high. This is when PMS (premenstrual syndrome) symptoms can show up: mood swings, irritability, breast tenderness, bloating, cravings, or feeling tearful. Not everyone gets PMS, and severity varies hugely.

Things people get wrong

"Everyone has a 28-day cycle." It's just an average. Plenty of people have shorter or longer cycles, and that's normal.

"You can't get pregnant on your period." It's unlikely but not impossible, especially if you have a shorter cycle. Sperm can survive for days, so sex during a period could potentially lead to pregnancy if ovulation happens earlier than expected.

"PMS is just an excuse." PMS is caused by real hormonal changes. The symptoms are physical and emotional, and for some people they're significant. Dismissing it isn't helpful or accurate.

Things people ask about the menstrual cycle

Why is my cycle irregular?

Irregular cycles are very common, especially in the first few years after your first period. Stress, illness, changes in weight or exercise, and hormonal fluctuations can all affect timing. If your cycle is consistently very short (under 21 days), very long (over 35 days), or you're missing periods altogether, it's worth seeing a GP.

How do I track my cycle?

Note the first day of each period in a diary, calendar, or period-tracking app. Over a few months, you'll see your pattern. This helps you predict when your period is due and spot any changes.

Does hormonal contraception change the cycle?

Yes — that's how it works. Most hormonal methods suppress ovulation, thin the lining, or both. The "period" you get on the pill (during the break week) isn't a real period — it's a withdrawal bleed caused by the drop in hormones. Some methods stop periods altogether.

When am I most fertile?

Around ovulation — roughly mid-cycle, but the exact day varies. The fertile window is about 6 days: the 5 days before ovulation (because sperm survive that long) and the day of ovulation itself.

The menstrual cycle is something that happens about once a month in girls' and women's bodies. It's the process where the body gets ready for a possible pregnancy, and then resets if pregnancy doesn't happen.

Here's roughly how it works. First, the period happens. This is when the soft lining inside the womb comes away and leaves the body as blood. This usually lasts a few days.

Then the body starts building up that lining again, making it thick and soft. At the same time, an egg starts getting ready inside one of the two small organs where eggs are stored.

About halfway through the cycle, the egg is released. This is called ovulation. If the egg meets a sperm, pregnancy can start. If it doesn't, the egg breaks down.

When the body realises there's no pregnancy, the lining comes away again. That's the next period. And the whole cycle starts over.

People often say the cycle is 28 days, but that's just an average. Anything from 21 to 35 days is normal. Many people's cycles aren't perfectly regular, especially in the first few years.

The hormones that control this process can also affect how you feel — your mood, energy, skin, and appetite can all change at different points in the cycle. This is normal.