Lo básico del ciclo

Hormones across your cycle

Revisado: 4 de octubre de 2026

Solo en inglés por ahora

Four main hormones run your cycle: FSH and LH from the brain, and oestrogen and progesterone from the ovaries. Here is what each one does, phase by phase.

Signals between brain and ovaries

Hormones are chemical messengers carried in your blood. Your cycle is run by signals passing between your brain and your ovaries. A part of the brain called the hypothalamus controls the pituitary gland. This small gland at the base of the brain makes two hormones, called FSH and LH, which act on the ovaries. The ovaries answer by making oestrogen and progesterone. These act on the womb (uterus) and also send signals back to the brain.

FSH: getting eggs ready

FSH stands for follicle-stimulating hormone. Early in the cycle, it makes several follicles in the ovaries grow. Follicles are small sacs that each hold an egg, and FSH helps get an egg ready for release. As the follicles grow, they send out signals that slow FSH down again. Later in life, as the ovaries release fewer eggs, the body makes more FSH. So a high FSH level is expected around perimenopause and after menopause.

Oestrogen: the first half

The growing follicles make oestrogen, so its level climbs through the first half of the cycle. Oestrogen rebuilds and thickens the womb lining (endometrium) after your period. It also makes the mucus from the neck of the womb (cervix) thinner and stretchier as ovulation gets close. For most of the cycle, oestrogen keeps FSH and LH in check. But when it reaches a high level just before ovulation, it does the opposite and triggers a big release of LH.

LH: the trigger for ovulation

LH stands for luteinising hormone. Like FSH, it is made by the pituitary gland. For most of the cycle it stays fairly low, then it rises quickly in what is called the LH surge. The surge starts about 36 hours before ovulation and peaks about 12 hours before. It makes the ripest follicle release its egg. Home ovulation tests work by detecting this rise in LH.

Progesterone: the second half

After ovulation, the empty follicle turns into the corpus luteum, a short-lived structure that makes large amounts of progesterone. Progesterone gets the womb lining ready for a fertilised egg to settle in (implantation). It also keeps FSH and LH low, so no new eggs ripen in the meantime. Doctors call this part of the womb's cycle the secretory phase.

When levels fall, a period starts

If there is no pregnancy, the corpus luteum stops making progesterone after about 10 to 12 days. Oestrogen and progesterone both drop to very low levels, and this tells the body to shed the womb lining as a period. With progesterone gone, the brain can make more FSH again, and a new group of follicles starts to grow. Because these hormones change through the month, a doctor may ask for a blood test at a certain point in your cycle.

This is general information. Hormone levels differ between people and across the month; a doctor or nurse can explain what any test results mean for you.