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What is the menstrual cycle?


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Menstruation is the monthly shedding of the lining of your uterus. Menstruation is also known by the terms menses, menstrual period, menstrual cycle, or period. Menstrual blood — which is partly blood and partly tissue from the inside of your uterus — flows from your uterus through your cervix and out of your body through your vagina.

Menstruation is driven by hormones. Hormones are chemical messengers in your body. Your pituitary gland (in your brain) and your ovaries (part of your reproductive system) make and release certain hormones at certain times during your menstrual cycle.

These hormones cause the lining of your uterus to thicken. This happens so that if a pregnancy occurs, an egg can implant into your uterine lining. Hormones also cause your ovaries to release an egg (ovulation). The egg moves down your fallopian tubes, where it waits for sperm. If a sperm doesn’t fertilize that egg, pregnancy doesn’t occur. The lining of your uterus breaks down and sheds. This is your period. The menstrual cycle prepares your body for pregnancy. If you are not pregnant, your hormones send a signal to your uterus to shed its lining. This becomes your period. Once you start your period, the cycle starts again.

A menstrual cycle is measured from the first day of your period to the first day of your next period. The average length of a menstrual cycle is 28 to 29 days, but every woman’s cycle is different. For example, teenagers might have cycles that last 45 days, whereas women in their 20s to 30s might have cycles that last 21 to 38 days.

Your first period is called menarche. In Western countries, the average age for a first period is 12 to 13, but it can start as early as nine and as late as 16.

Your last period is called menopause. In Australia, the average age for women to reach menopause is 51 to 52. Some women might reach menopause as late as 60.

Phases of the menstrual cycle
There are four main phases of the menstrual cycle.

1. Menstruation
Menstruation is commonly known as a period. When you menstruate, your uterus lining sheds and flows out of your vagina. Your period contains blood, mucus, and some cells from the lining of your uterus. The average length of a period is three to seven days.

Sanitary pads, tampons, period underwear, or menstrual cups can be used to absorb your period. Pads and tampons need to be changed regularly (preferably every three to four hours) and menstrual cups should be changed every eight to 12 hours.

2. The follicular phase
The follicular phase starts on the first day of your period and lasts for 13 to 14 days, ending in ovulation. The pituitary gland in the brain releases a hormone to stimulate the production of follicles on the surface of an ovary. Usually, only one follicle will mature into an egg. This can happen from day 10 of your cycle. During this phase, your uterus lining also thickens in preparation for pregnancy.

3. Ovulation
Ovulation is when a mature egg is released from an ovary and moves along a fallopian tube towards your uterus. This usually happens once each month, about two weeks before your next period. Ovulation can last from 16 to 32 hours.

It is possible to get pregnant in the five days before ovulation and on the day of ovulation, but it’s more likely in the three days leading up to and including ovulation. Once the egg is released, it will survive up to 24 hours. If sperm reaches the egg during this time, you may get pregnant.

4. The luteal phase
After ovulation, cells in the ovary (the corpus luteum), release progesterone and a small amount of estrogen. This causes the lining of the uterus to thicken in preparation for pregnancy.

If a fertilized egg implants in the lining of the uterus, the corpus luteum continues to produce progesterone, which maintains the thickened lining of the uterus.

If pregnancy does not occur, the corpus luteum dies, progesterone levels drop, the uterus lining sheds and the period begins again.