Menstrual cycle

Illustration: Les publications du Québec

Your body prepares for pregnancy during every menstrual cycle. Menstruation is a stage of the menstrual cycle. Menstrual cycles begin at puberty around the age of 12 and continue, on average, until menopause at around age 51.

To determine the length of your menstrual cycle, count the number of days from the beginning of your period to the day before your next period starts. Menstrual cycles can last anywhere from 21 to 35 days, but are usually between 28 and 30 days long.

During a menstrual cycle, your body goes through a number of changes. Many interactions take place between your brain and your pituitary gland, a hormone-secreting organ. These interactions trigger the release of hormones that stimulate ovulation, which in turn prepares your body for fertilization.


Women are born with all the eggs they will ever have. They have about 400,000 eggs at puberty, and by menopause, all of them are gone.

Female reproductive system

Illustration: Bertrand Lachance

Ovulation occurs when an ovary releases an egg. Once an egg is released, it is drawn into the fallopian tube. If it comes into contact with sperm and is fertilized, a new cell is formed and starts to multiply. The new cells travel down the fallopian tube to implant themselves in the uterus and form an embryo.

To estimate when you will ovulate, count backwards 14 days from the end of your menstrual cycle. Women with regular 28-day cycles usually ovulate around the 14th day of their cycle. For women with irregular cycles, however, it is more difficult to predict the day or period of ovulation.

Fertile period (or ovulation period)

Since ovulation does not always occur on the expected day, we talk about the fertile period or ovulation period. This is when a woman is most likely to ovulate. If a man and a woman have intercourse during the woman’s fertile period and she has a regular menstrual cycle, there is a one in four chance (at age 20) and a one in twenty chance (at age 40) that the woman will become pregnant.

If the egg is not fertilized by a sperm, the glands in the brain will stop producing hormones. This triggers menstruation, and the cycle starts all over again.

Menstrual cycle: The time between two menstrual periods.

Fertilization: When a sperm penetrates an egg.

Egg: Cell produced by the ovary. When the egg is fertilized, a baby may begin to form.

Sperm: Cell produced by the man. When it fertilizes an egg, a baby may begin to form.

Embryo: Name given during the first full 10 weeks of pregnancy to the human being developing in the mother’s abdomen.