The menstrual cycle is controlled by the female hormones oestrogen and progesterone. These hormones are responsible for maintaining fertility by producing eggs and preparing the uterus (womb) for the implantation of a developing embryo. When pregnancy does not occur the lining of the uterus is shed which results in your monthly bleeding.
The menstrual cycle starts from the first day of a woman's period to the day before her next period. The length of the menstrual cycle varies from woman to woman, although the average is 28 days. Regular cycles that are shorter or longer than this, from 23 to 35 days, are considered normal.
What Is Considered ‘Normal’?
Not all girls are the same, with some maturing earlier or later than others. Likewise, some women remain fertile into their 60s while others enter menopause as early as their late 30s. There are, however, some average guidelines regarding the menstrual cycle, namely:
- The average age that a girl gets her first period is around 12 years. It can vary from 8 to 16 years.
- Most girls menstruate within 2-3 years of the first signs of puberty (developing breasts and growing pubic hair).
- The average age when periods stop (the age of menopause in the United Kingdom) is 51.
- The average length of a period (menstruation) is 2 to 7 days.
- Women lose at least 20 to 90ml (about 2-5 tablespoons) of blood in a period.
- Women with a regular, 28-day cycle will ovulate (release an egg) on day 14. However, ovulation can take place between days 10 to 16.
- Between the ages of 12 and 52, a woman will have approximately 480 periods.
What Is Considered ‘Abnormal’?
Although there is a wide range of acceptable timeframes regarding menstruation, certain irregularities could indicate an underlying health problem that requires further investigation. These irregularities may include:
- If your periods have not started by the age of 15, or if you have no signs of puberty by the age of 13 you should see a doctor.
- Periods that occur less than 21 days or more than 35 days apart.
- Not having a period for three months.
- Menstrual flow that is much heavier or lighter than usual.
- Menstrual bleeding that lasts longer than seven days.
- Periods that are accompanied by severe pain, cramping, nausea or vomiting.
- Bleeding or spotting that happens between periods.
What Is Menstruation?
Menstruation, also known as menses, menstrual period, menstrual cycle or period, is when the lining of the uterus (womb) is shed every month. During your period, menstrual blood, which contains both blood and uterine tissue, flows from your uterus, through your cervix and out through your vagina. The first day of your period is the first day of your menstrual cycle.
Menstruation is governed by various hormones which are released by your pituitary gland (in your brain) and ovaries at different times in your cycle. These hormones cause the endometrium (lining of your womb) to thicken which allows an egg to implant. They also cause ovulation to occur, which is the release of an egg from the ovaries. During each menstrual cycle, an egg moves down your fallopian tubes, where it waits for sperm so that conception (fertilisation) can take place. When fertilisation does not take place, the lining of the uterus breaks down and sheds (your period). The cycle then repeats itself.
How Does The Menstrual Cycle Work?
The menstrual cycle has four stages. These four stages are caused by the rise and fall of the hormones progesterone and oestrogen which trigger the stages in your menstrual cycle. These hormones cause your reproductive organs (uterus, ovaries and fallopian tubes) to respond in certain ways. The four stages of the menstrual cycle include:
The Menses Phase
The first day of a woman's period is day 1 of the menstrual cycle. This phase normally lasts from day 1 to day 5 and relates to the time when you are menstruating. This is when the lining of your uterus is shed if pregnancy has not occurred.
The Follicular Phase
This phase usually takes place from days 6 - 14. During this phase, the level of oestrogen rises, which causes the endometrium to grow and thicken. At the same time another hormone, known as follicle-stimulating hormone (FSH), causes follicles to grow in your ovaries. Between days 10 to 14, one of these follicles will form a fully mature egg (ovum).
Ovulation normally takes place in the middle of your menstrual cycle (around day 14 in a 28-day cycle). A hormone called luteinizing hormone (LH) causes your ovary to release its egg. The release of an egg is known as ovulation.
The Luteal Phase
In a typical 28-day cycle, this phase lasts from about day 15 to day 28. Once an egg is released from your ovary it travels through your fallopian tubes and into your uterus. Progesterone levels start to rise to help prepare your uterine lining for pregnancy. If the egg becomes fertilised by sperm (conception) and attaches itself to your uterine wall (implantation), you become pregnant. If conception does not occur, hormone levels drop, and the thick lining of your uterus is shed in the form of a period.
Do Your Periods Change Over Time?
Your menstrual cycle will normally change from your teen years until you start approaching menopause in your 40s or 50s. It is very common to have longer cycles or heavier periods (blood flow) when you first get your period. It often takes around three years for your menstrual cycle to normalise.
Once you reach your 20s, you should be having more consistent and regular periods. Once you start reaching the age of menopause (in your 40s and 50s), your periods will change again and become more irregular (this is also known as perimenopause).
If you have had a baby or are lactating (breastfeeding), your changing hormone levels will cause the pattern of your periods to change for some time.