The Hormones Behind Your Menstrual Cycle, Phase by Phase

The short answer: Your menstrual cycle is run by five main hormones. FSH and LH come from the pituitary gland in your brain; oestrogen, progesterone and a little testosterone come mostly from your ovaries. In the follicular phase, FSH grows an egg and oestrogen rises. A surge of LH triggers ovulation. In the luteal phase, progesterone rises to prepare the womb lining. If you're not pregnant, oestrogen and progesterone fall, and your period starts. These shifts also explain changes in mood, energy and discharge.

Your cycle is a conversation between your brain and your ovaries

A menstrual cycle isn't controlled by one organ. It's a chain of messages. A part of your brain called the hypothalamus sends a signal to the pituitary gland, a pea-sized gland just below it. The pituitary then releases two hormones into your blood that travel to your ovaries and tell them what to do.

The ovaries respond by growing an egg and making oestrogen and progesterone, which travel back to the brain and change the next signals. It's a feedback loop, like a thermostat.

This is why so many things can affect your period. Stress, illness, a big change in weight or a thyroid problem can all disturb the messages at the brain end, even when your ovaries and womb are perfectly healthy.

Day 1 of your cycle is the first day of proper bleeding. The average cycle is around 28 days, but anything from 21 to 35 days is considered normal. If you're not sure how to count yours, our guide on how to calculate your next period date has worked examples.

The five hormones you need to know

FSH (follicle-stimulating hormone)

Made by the pituitary gland. As its name says, it stimulates follicles, the tiny fluid-filled sacs in your ovaries that each hold an immature egg. FSH rises at the start of your cycle and wakes up a group of follicles. Usually one of them takes the lead and matures fully.

LH (luteinising hormone)

Also made by the pituitary gland. It stays fairly low for most of the cycle, then shoots up in a sudden "LH surge" in the middle. This surge is the trigger that makes the ripest follicle burst and release its egg.

Oestrogen

Made mainly by the growing follicles in your ovaries. Oestrogen thickens the lining of the womb (the endometrium) after your period, makes cervical mucus wetter and more slippery around ovulation, and plays a part in bone strength, skin and mood. It climbs through the first half of the cycle and peaks just before ovulation.

Progesterone

After ovulation, the empty follicle turns into a small temporary gland called the corpus luteum, which makes progesterone. Progesterone makes the womb lining soft and ready for a fertilised egg to settle in. It also raises your body temperature slightly and thickens cervical mucus. Progesterone is the main hormone of the second half of your cycle.

Testosterone

Women make testosterone too, in much smaller amounts than men. It comes from the ovaries and the adrenal glands (small glands above the kidneys), and much of it is converted into oestrogen. In women, testosterone supports sex drive, energy, mood, and bone and muscle strength. Levels stay fairly steady through the cycle. When they're too high, it can cause irregular periods, acne, extra facial or body hair and thinning scalp hair, which is one of the hallmarks of PCOS. Our guide to hair fall, facial hair and your hormones explains this side in detail.

What happens in each phase of your cycle

Doctors usually describe four phases. Two of them overlap: your period is actually the first part of the follicular phase. The timings below are for a 28-day cycle, so shift them if yours is shorter or longer.

Menstrual phase: about days 1 to 5

Oestrogen and progesterone are at their lowest. Without these hormones to hold it in place, the womb lining breaks down and leaves the body as your period. Most people bleed for three to five days, and up to seven is still normal. At the same time, low hormone levels tell the brain to start raising FSH, so the next cycle is already beginning.

Follicular phase: about days 1 to 13

FSH encourages several follicles to grow. As they grow, they make oestrogen, which rebuilds and thickens the womb lining. By around days 10 to 14, one follicle has become dominant and its egg is nearly ready. This phase is the one that varies most in length, which is the main reason cycle lengths differ from person to person and from month to month.

Ovulation: around day 14

High oestrogen tells the brain to release a burst of LH. Within about a day or so of the surge, the dominant follicle releases its egg into the fallopian tube. This is your most fertile time. Some women feel a twinge on one side of the lower tummy, called mittelschmerz, and many notice clear, stretchy discharge.

Luteal phase: about days 15 to 28

The corpus luteum makes progesterone, plus some oestrogen. The lining becomes thick and ready. The luteal phase tends to be more consistent in length than the follicular phase, usually around two weeks.

If an egg is fertilised and implants, the pregnancy hormone hCG keeps the corpus luteum going, so progesterone stays high and no period comes. This is the hormone a pregnancy test picks up. If there's no pregnancy, the corpus luteum shrinks after about 10 to 14 days, oestrogen and progesterone drop sharply, and your period begins. Then the cycle starts again.

Your hormones phase by phase: at a glance

Treat the "how you may feel" column as a common pattern, not a rule. Many women notice very little change.

How your hormones explain PMS, discharge, energy and mood

Many changes you notice through the month are your hormones at work. Each has its own guide, so here's the short version.

• PMS: the fall in oestrogen and progesterone before your period seems to affect brain chemicals like serotonin, causing bloating, sore breasts and mood dips in some women. Read more in PMS: symptoms, causes and what actually helps.
• Discharge: rising oestrogen makes discharge wetter and more slippery towards ovulation, and progesterone makes it thicker or drier afterwards. Our guide to normal vaginal discharge through the month shows the full pattern.
• Energy and appetite: many women feel more energetic in the follicular phase and hungrier in the days before a period. Our guide on what to eat in each phase of your menstrual cycle has Indian meal ideas for each stage.

Staying comfortable across the month

Because your period can arrive a day or two earlier or later than expected, it helps to be ready in the late luteal phase. A period panty has a built-in absorbent core with one absorbency for every flow, so you can wear it on days you think bleeding might start and through your period itself, for up to 8 hours depending on flow. Around ovulation, when discharge is heaviest, a panty liner keeps you feeling fresh.

What can throw your hormones off balance

When the brain-to-ovary conversation is disturbed, ovulation can be delayed or skipped. That usually shows up as a late, missed, lighter or heavier period. Common reasons include:

• Stress: severe stress, such as exams, a family illness or a big move, can delay the signals that trigger ovulation.
• Big weight changes or under-eating: the brain reads sudden weight loss or not eating enough as a sign that it's not a safe time for pregnancy and turns down FSH and LH.
• Very intense exercise: especially when training hard without eating enough to fuel it.
• Thyroid problems: an underactive or overactive thyroid can make periods heavier, lighter or irregular. Our guide to thyroid and your periods explains the tests and treatment.
• PCOS: higher androgen (testosterone-type hormone) levels and insulin resistance can stop follicles maturing, so ovulation happens less often. See PCOS explained, and our guides to exercise for PCOS and long-term PCOS health risks.
• High prolactin: prolactin is the hormone that makes breast milk. It's naturally high while breastfeeding, which is why periods often stay away for months after birth. Raised prolactin at other times can also stop periods.
• Life stages: cycles are often irregular in the first few years after periods start, and again in the years before menopause. Our perimenopause guide covers the later stage.

For a fuller list of causes and what helps, read our guide to irregular periods.

Hormonal contraception in simple terms

Hormonal contraceptives work by using man-made versions of your own hormones to change the cycle.

• The combined pill: contains oestrogen and a progestogen (a man-made form of progesterone). It stops your ovaries releasing an egg each month. The bleed you get in the pill-free break or dummy-pill week is like a period, but it's caused by the drop in pill hormones rather than by your own cycle.
• Progestogen-only methods: the progestogen-only pill, injection, implant and hormonal coil mainly work by thickening cervical mucus and thinning the womb lining, and some also stop ovulation. They can make bleeding lighter, irregular or stop altogether.

Because the pill smooths out hormone swings, your doctor may suggest it for problems like painful periods, PMS or PCOS, not just for contraception. When you stop hormonal contraception, your own cycle usually returns, though it can take a few months to settle.

Hormone tests and when doctors use them

You don't need hormone tests if your periods are regular and you feel well. Doctors use them to find a cause when something has changed. Timing matters, because hormone levels vary so much through the month.

• Pregnancy test (hCG): always the first step for a late or missed period if there's any chance of pregnancy.
• FSH and LH: often checked early in the cycle, around days 2 to 5, if periods are irregular or have stopped, or to look at how the ovaries are working. A high FSH can be a sign of approaching menopause.
• Progesterone: checked in the second half of the cycle, about a week before your next period is due, to see whether you've ovulated. The NHS notes that the timing depends on how regular your periods are.
• Testosterone: if PCOS is suspected, for example with irregular periods, acne or extra hair growth.
• Prolactin and thyroid tests (TSH): common early checks when periods are irregular or missing.

Avoid booking large "hormone panels" on your own, as a single number on the wrong day can cause needless worry. Ask your doctor which tests you need and on which day of your cycle to have them.

Myths about hormones and your cycle

• "You must eat and exercise differently in every phase." Some apps promote strict "cycle syncing". Noticing your own energy patterns is useful, but there's little good evidence that rigid phase-by-phase rules improve your health. A steady, balanced routine all month is what matters most.
• "Hormone-balancing teas and supplements will fix your cycle." No tea, powder or supplement can reset your hormones. If your cycle has changed, there's usually a reason worth finding, such as a thyroid problem or PCOS.

See a doctor if

• Your cycle is often shorter than 21 days or longer than 35 days
• You've missed three periods in a row and you're not pregnant
• Your periods are very heavy, last longer than 7 days, or you bleed between periods
• You have acne, extra facial or body hair, or hair thinning along with irregular periods
• PMS is affecting your work, studies or relationships
• You've been trying to get pregnant for a year without success, or sooner if you're in your mid-30s or older
• Your periods haven't started by age 15

Get urgent help by calling 112 or 108, or going to the nearest hospital emergency department, if you have very heavy bleeding with dizziness or fainting, or severe one-sided tummy pain and could be pregnant.

Frequently asked questions

Which hormone causes your period to start? No single hormone switches it on. Your period starts when progesterone and oestrogen fall at the end of the luteal phase, because the womb lining can no longer be maintained.

Which hormone triggers ovulation? A sudden surge of LH from the pituitary gland, set off by high oestrogen levels, triggers the release of the egg.

Can stress really change my hormones? Yes. Severe stress can delay the brain signals that trigger ovulation, which can push your period later or make you skip one.

Do I ovulate on day 14? Only if your cycle is around 28 days and very regular. Ovulation usually happens roughly two weeks before your next period, so in a 35-day cycle it may be around day 21.

Does the pill stop my hormones? The combined pill replaces your natural rise and fall of hormones with steady pill hormones and stops ovulation. Your own cycle usually returns after you stop taking it.

Can a blood test tell me if my hormones are "balanced"? Not on its own. Hormone levels change every day of the cycle, so tests are only useful when a doctor orders the right ones on the right day.

Related reading

• PMS: Symptoms, Causes and What Actually Helps
• Normal Vaginal Discharge: What to Expect Through the Month
• Thyroid and Your Periods: How Thyroid Problems Affect Your Cycle
• What to Eat in Each Phase of Your Menstrual Cycle: A Complete Nutrition Guide

This article is for general information and doesn't replace advice from your doctor.

Sources: Cleveland Clinic: Menstrual cycle (normal menstruation), Cleveland Clinic: Testosterone, NHS: What is the combined pill?, NHS: Diagnosis of infertility, NHS: PMS (premenstrual syndrome)