Lo básico del ciclo
How the menstrual cycle works
Revisado: 4 de octubre de 2026
Solo en inglés por ahoraEach cycle, your body gets ready for a possible pregnancy and then resets with a period. Here is the big picture of what happens from one period to the next.
What the cycle is for
The menstrual cycle is a pattern of changes that repeats about once a month. It is run by hormones, which are chemical messengers in your blood. Each cycle, your body gets ready for a possible pregnancy. An egg ripens and is released, and the inner lining of the womb (uterus) grows thick. If you do not get pregnant, hormone levels fall and the lining comes away. That bleeding is your period (menstruation). Periods usually start at around age 12 and continue until menopause, which most often happens in the mid-40s to mid-50s.
How long a cycle lasts
Day 1 of a cycle is the first day of period bleeding. The cycle runs until the day before your next period starts. Most adult cycles last somewhere between about 21 and 38 days. The average is around 28 days, but few people have exactly 28 every time, and the length can change from month to month. The period itself usually lasts 2 to 7 days, often about 5.
First half: an egg ripens
During and after your period, the brain sends signals to the ovaries. Small sacs in the ovaries called follicles, each holding an egg, start to grow. As they grow, they make the hormone oestrogen. Oestrogen rebuilds the womb lining after your period and makes it thicker. It also makes the mucus from the neck of the womb (cervix) thinner and stretchier. Usually one follicle grows bigger than the rest, and its egg is the one that will be released.
Ovulation: the egg is released
When oestrogen is high, the brain releases a burst of luteinising hormone (LH). This makes the leading follicle release its egg, which is called ovulation. It usually happens about 10 to 16 days before the next period, so the exact day depends on how long your cycle is. The egg moves into a fallopian tube, which carries it towards the womb. Pregnancy is most likely from sex in the days before and around ovulation, because sperm can survive in the body for up to 7 days.
Second half: the womb gets ready
After ovulation, the empty follicle turns into a small structure called the corpus luteum. It makes the hormone progesterone, which keeps the womb lining thick and ready for a fertilised egg to settle in. If no pregnancy happens, the corpus luteum stops working. Oestrogen and progesterone then drop to low levels, and this tells the body to shed the lining. Your period starts, and a new cycle begins on its first day.
When to talk to a doctor or nurse
Get advice if your cycles are regularly shorter than 24 days or longer than 38 days. Also get advice if you have no period for 3 months and are not pregnant or breastfeeding. If pain or bleeding stops you doing everyday things, get it checked, because treatment can help. Bleeding between periods, after sex or after menopause should always be checked by a doctor. Get urgent help if you bleed very heavily and feel dizzy or faint, or have sudden, severe pain in your lower tummy. Logging your periods in Ciclo can help you notice changes like these.
This is general information about a typical cycle. Every body is different; if something about your cycle worries you, talk to a doctor or nurse.