Oestrogen

Oestrogen (spelled "estrogen" in American English) is a hormone that plays a major role in puberty, the menstrual cycle, and pregnancy. It's often called a "female hormone," but everyone produces it — people with ovaries just produce a lot more. It's responsible for many of the physical changes that happen during puberty in people with ovaries, and it keeps the menstrual cycle running throughout the reproductive years.

• Oestrogen is a sex hormone mainly produced by the ovaries.
• It drives puberty changes like breast development, wider hips, and the start of periods.
• It controls the first half of the menstrual cycle, building up the uterus lining before ovulation.
• Everyone has some oestrogen — people with ovaries have more, people with testes have less.
• It's also used in hormonal contraception (the combined pill, patch, and vaginal ring all contain oestrogen).

What it does during puberty

When puberty starts in people with ovaries, rising oestrogen levels trigger a chain of physical changes:

• Breast tissue begins to develop
• Hips widen as the pelvis grows
• Body fat distribution shifts (more on hips, thighs, and bum)
• The uterus and vagina grow
• The growth spurt kicks in (oestrogen is part of what triggers it — and also what eventually stops it, by causing growth plates to close)
• Eventually, the menstrual cycle begins

These changes happen gradually over several years. Oestrogen levels don't just switch on and stay level — they rise and fall throughout each menstrual cycle, which is why you might feel different at different points in your cycle.

What it does in the menstrual cycle

Oestrogen is the dominant hormone in the first half of the cycle (the follicular phase). After your period, oestrogen levels rise steadily. This does two things: it tells the uterus lining to thicken and build up (getting ready for a possible pregnancy), and it triggers the release of LH, which causes ovulation.

After ovulation, oestrogen drops and progesterone takes over for the second half of the cycle. If there's no pregnancy, both hormones fall, the lining sheds, and you get your period.

This is why many people feel more energetic, clear-headed, and upbeat in the first half of their cycle when oestrogen is rising — and more tired, moody, or flat in the second half when it drops.

Oestrogen and contraception

The combined pill, the patch, and the vaginal ring all contain synthetic oestrogen (alongside progestogen). The oestrogen helps prevent ovulation and regulates bleeding. Not everyone can use oestrogen-based contraception — people with certain health conditions (like a history of blood clots or migraines with aura) are usually advised to use progestogen-only methods instead. [CHECK: current FSRH contraindications for oestrogen-containing contraception]

Things people get wrong

"Oestrogen is only a female hormone." Everyone has oestrogen. In people with testes, it plays a role in bone health, brain function, and sperm development. The amount is different, but it's not absent.

"More oestrogen means more feminine." Oestrogen levels vary naturally from person to person. Having higher or lower levels doesn't make you more or less of anything — it's just biology.

"Oestrogen only matters during puberty." It's active throughout life. It regulates the menstrual cycle, supports bone density, affects mood and energy, and plays a role in skin and brain health well beyond puberty.

Things people ask about oestrogen

Why do I feel different at different times of the month?

Partly because oestrogen levels rise and fall across your cycle. When oestrogen is high (mid-cycle), many people feel more energetic and positive. When it drops (before your period), you might feel lower, more tired, or more emotional. These shifts are real and hormonal — not imagined.

Does oestrogen affect my mood?

Yes. Oestrogen influences serotonin and other brain chemicals that affect mood. This is one reason PMS happens — the drop in oestrogen (and progesterone) before your period can genuinely affect how you feel.

What happens to oestrogen on the pill?

The combined pill provides a steady dose of synthetic oestrogen, which overrides your natural cycle. This stops the usual hormonal ups and downs — which is why some people find the pill evens out their mood, while others feel flatter or experience different mood changes.

Is oestrogen used in gender-affirming treatment?

Yes. Trans women and some non-binary people may take oestrogen as part of hormone therapy to develop physical characteristics like breast growth, softer skin, and fat redistribution. This is prescribed and monitored by specialist clinics. [CHECK: current NHS pathway for accessing gender-affirming hormones]

Oestrogen is a hormone — a chemical that travels through the blood and tells parts of the body what to do. It's mainly made by the organs where eggs are stored inside girls' and women's bodies.

During puberty, oestrogen causes a lot of the changes that happen to girls' bodies. Breasts start to develop. Hips get wider. The body shape changes. Eventually, periods start.

Oestrogen also controls the first half of the monthly cycle. After a period, it tells the body to build up a soft lining inside the womb, getting ready in case a pregnancy starts. It also helps trigger the release of an egg.

The levels of oestrogen go up and down throughout the month. This is part of why you can feel different at different times — more energetic some weeks, more tired or emotional at others. These feelings are real and caused by the hormone changing.

Everyone has some oestrogen, not just girls and women. Boys and men have smaller amounts that help with things like bone strength and brain function.

Some types of birth control contain a version of oestrogen. It helps stop the body from releasing an egg, which prevents pregnancy.