The menstrual cycle: What is menstruation?

PDF download is not available for Arabic and Urdu languages at this time. Please use the browser print function instead

Learn about menstruation (periods) and what makes up a normal menstrual cycle.

Key points

  • Menstruation (periods) is experienced by people with female reproductive body parts.
  • It is the body’s monthly cycle of preparing itself for a possible pregnancy: an egg is released by the ovaries and, if it remains unfertilized throughout the cycle, the uterine lining is shed.
  • Most people who menstruate begin during puberty when they are between 9-15 years old.
  • The average time between periods (menstrual cycle length) is approximately 28 days.

What is menstruation?

Menstruation, periods or the menstrual cycle, is experienced by people with female reproductive body parts and is the body’s monthly process of preparing itself for a possible pregnancy. Approximately once a month, the ovaries release an egg (or ovum) through a process called ovulation. During this cycle, the uterus prepares for a possible pregnancy by building up a thick lining called the endometrium. If the released egg is not fertilized by sperm, no pregnancy begins, and the uterus then sheds its lining resulting in menstrual bleeding. This bleeding is often called a “menses,” or “period.” You might also hear people talk about this bleeding using other nicknames and phrases like “Aunt Flo,” the “Time of the Month,” a person’s “Moon Time,” being on/having “Your days”, “Little Red Riding Hood is coming,” or the “Little strawberry.”

At what age do you get your period?

The normal range for getting your period is between 9-15 years of age. In Canada, the average age is usually between ages 12 and 13 years.

The age at which your period starts is called “menarche”. Menarche occurs towards the end of puberty, and it indicates that your body has the ability to become pregnant. If you are sexually active and having vaginal sex, it is possible to get pregnant around the time of your first period.

Menarche occurs at different ages. It is impossible to predict exactly when menarche will occur, but it is closely tied to the timing of puberty. The first sign of puberty is usually the start of breast development, and menarche often happens about two years after breast development begins. It is not exactly known what triggers menarche, but several factors can influence when it happens. These factors include your race, nutrition status, body composition (including weight), genetics, any health conditions you might have, and environmental factors.

Once you begin menstruating, it can take some time before your cycles become regular. Some people have very predictable cycles throughout their lifetime, and others have unpredictable cycles. The regularity of your cycle can also change over time. You will continue to menstruate until menopause (typically until around the age of 50).

What causes menstruation?

Menstruation is caused by a complex interaction between the hormones produced in your brain and the hormones produced in your ovaries. Your brain produces hormones called follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which then stimulate the ovaries to produce the hormones estrogen and progesterone.

The interaction between all of these hormones (FSH, LH, estrogen, progesterone) determines how your uterus changes during your menstrual cycle.

What is a normal cycle length?

A cycle length is measured from the first day of bleeding (Day 1 of one menstrual period) to the first day of the next menstrual period. There is a wide range of normal cycle lengths. For teenagers, a normal menstrual cycle can be anywhere between 21-45 days. The average menstrual cycle length is approximately 28 days.

In the first 1-2 years following your first period, it is very common and normal to have irregular cycles. It is also common to have anovulatory cycles (where no ovulation occurs). In fact, in the first year after your first period, up to 80% of your menstrual cycles may be anovulatory. Since you won’t be able to tell if you’ve had an ovulatory or anovulatory cycle, it is important to always use contraception if you would like to prevent pregnancy.

Once you start your period, it is a good idea to keep track of when your cycles happen, either by using a calendar or an app. Because the average cycle length is 28 days, many apps or calendars designed for “period tracking” are often based on a 28-day cycle. If your menstrual cycle is not 28 days and you are using an app or calendar based on a 28-day cycle, the predictions of when your period will occur may be inaccurate.

What happens during a normal menstrual cycle?

The menstrual cycle can be divided into three major phases: the follicular phase, ovulatory phase, and luteal phase.

The timing and events for each phase below is based on a cycle length of 28 days:

Day(s) of cycleWhat happensOutcome
Follicular phase
Days 1-14
  • Day 1 is the first day of bleeding.
  • Bleeding continues for 3-8 days (menstrual period).

FSH stimulates development of follicles in the ovaries.

  • Each follicle contains an egg.
  • One of these follicles becomes dominant, forming a mature egg.
  • The other follicles do not develop further, and eventually recede.

Higher levels of estrogen stimulate the uterus to build up its lining for potential pregnancy.

  • This triggers the release of high levels of LH (known as the “LH surge”).
The LH surge causes ovulation.
This phase ends with ovulation.
Ovulatory phase
Day 14The LH surge causes the dominant follicle in the ovary to release a mature egg (called ovulation).
  • The egg travels from the ovary to the fallopian tube, where the egg can become fertilized if sperm are present.

If the egg does not become fertilized, it disintegrates.

The follicle that released the egg becomes a structure called the “corpus luteum.”

  • This helps to produce progesterone.
Luteal phase
Days 14-28

The corpus luteum produces progesterone, which helps keep the uterine lining thick for a fertilized egg.

  • Without a fertilized egg, the corpus luteum eventually disintegrates and stops producing progesterone.

Lower progesterone levels trigger the uterine lining to shed.

Bleeding and shedding of the uterine lining begin (menstrual period).
Last updated: July 19th 2022