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What is a normal menstrual period?

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A normal menstrual period is triggered approximately once a month by a drop in progesterone levels in the second half of the menstrual cycle, after ovulation.  Ovulation occurs about halfway through an average menstrual cycle and is when one of the ovaries releases an egg which travels down the fallopian tubes in readiness for being fertilised and implanted in the uterus.

Menstrual Cycle and Moon

There can be a lot of variance between what is normal for one woman and what is normal for another, as we are all unique.

Period Length

A normal period generally lasts between 2 and 7 days, with most lasting between 3 and 5, and results in the loss of around 30 millilitres of fluid and tissue, mostly blood, mucus and degraded endometrial tissue.  A menstrual cycle is considered to begin the first day of bleeding.  This is called cycle day 1 or CD 1.

Cycle Length

Generally though, a normal menstrual cycle is between 22 and 36 days in length, measured from the first day of your last menstrual period to the first day of your next one.  The average is 28 days, the same as the lunar month and more women tend to ovulate around the time of the full moon and menstruate around the time of a new moon.  Research into this phenomenon and the regulatory effect of a light regimen replicating that of the moon was done by the US Air Force last century culminating in a patent being granted in 2002 and licensed to a company called Parhelion Labs who then developed a device called the “Luness” designed to exert a regulatory effect on the female reproductive cycle through a precisely dosed night-time light regimen.

Onset of menstruation

Most women will get their first period, an event termed menarche, around the age of 12.5 years of age, but it can happen anywhere between 8 and 15 years age.  Women with PCOS tend to have an earlier menarche than women without the condition.  The more weight a child gains during childhood, the earlier her menarche is likely to be.  Menarche usually occurs about 2 years after the breasts begin to develop.

The follicular phase

In the early part of the cycle, from around the end of the period, the ovaries start producing oestrogen, which causes the lining of the uterus, the endometrium, the grow and thicken, in preparation for receiving a fertilised egg.  Whilst this is happening, the pituitary gland is secreting a slightly higher amount of follicle stimulating hormone or FSH which directs a handful of follicles in the ovary to grow and for one of them to become dominant.

The follicular phase culminates in a surge of follicle stimulating hormone or FSH and luteinising hormone or LH from the pituitary gland in the brain, triggered by the peak in oestrogen levels, which causes the ovarian follicle, which contains the matured egg or oocyte, to rupture, releasing the egg in an event termed ovulation.


Oestrogen levels peak around the time of ovulation and then start to decrease, as progesterone levels start to increase, produced by the corpus luteum or the ruptured follicle which held the egg prior to ovulation.  Most women ovulate between 12 and 14 days before the start of a period.  This period of time doesn’t vary much, between different women or different cycles in the same woman.  Some women have a short luteal phase, or luteal phase defect, which is common in PCOS and can contribute towards difficulty conceiving.  If a woman has a ‘regular’ 28 day cycle, then she most likely ovulates on or around day 14 of her cycle, making her fertile around days 11 – 15.  If a woman has a short cycle of just 22 days, then she probably ovulates just a few days after her period ends making her fertile around days 6 – 10 and possible even during her period.  If a woman has a long cycle, for instance 36 days, she would be likely to ovulate around day 22, making her fertile around days 19 – 23.

The luteal phase

After ovulation, the corpus luteum starts producing progesterone, the ‘pregnancy hormone’ which is essential for sustaining a pregnancy in the first trimester.  If fertilisation of the egg does not occur, then the corpus luteum produces less and less progesterone from around day 22 with levels falling sharply from around day 26.  This drop in progesterone levels signifies to the body that pregnancy has not occurred and to shed the lining of the uterus, ready for another cycle.  Throughout most of the luteal phase the hormones FSH and LH are very low, but towards the end of the cycle they both increase a little to begin preparing the ovaries for another cycle.

 How much blood should you lose in a normal menstrual period?

A period is considered heavy if there is a loss of greater than 80 milliliters of menstrual flow over the entire period.  A gram of blood is roughly equivalent to a milliletre as the specific gravity of blood is around 1.05 – 1.06.  Using a menstrual cup is an easy way to know exactly how much mesntrual fluid you are losing, or the following estimates can be used for pads and tampons.

Junior/light/mini absorbency – less than 6 grams (1 teaspoon equals about 5 grams)

Regular absorbency – 6 to 9 grams

Super absorbency – 9 to 12 grams

Super plus absorbency – 12 to 15 grams

Ultra absorbency – 15 to 18 grams


The Mansfield-Voda-Jorgensen Menstrual Bleeding Scale can give you indication of what category your menstrual loss lies in:

1. Spotting. A drop or two of blood, not even requiring sanitary protection, though you may prefer to use one.

2. Very light bleeding. You need to change the least absorbent tampon or pad one or two times per day, though you may prefer to change more frequently.

3. Light bleeding. You need to change a low or regular absorbency tampon or pad two or three times per day, though you may prefer to change more frequently.

4. Moderate bleeding. You need to change a regular absorbency tampon or pad every three to four hours, though you may prefer to change more frequently.

5. Heavy bleeding. You need to change a high absorbency tampon or pad every three to four hours, though you may prefer to change more frequently.

6. Very heavy bleeding or gushing. Protection hardly works at all; you need to change the highest absorbency tampon or pad every hour or two.

More Information:

ACOG Committee on Adolescent Health Care. (2006) ACOG Committee Opinion No. 349, November 2006: Menstruation in girls and adolescents: using the menstrual cycle as a vital sign. Obstetrics and gynecology, 108(5), 1323-8. PMID: 17077267

Mansfield PK, Voda A, & Allison G. (2004) Validating a pencil-and-paper measure of perimenopausal menstrual blood loss. Women’s health issues : official publication of the Jacobs Institute of Women’s Health, 14(6), 242-7. PMID: 15589775

Treloar AE, Boynton RE, Behn BG, & Brown BW. (1967) Variation of the human menstrual cycle through reproductive life. International journal of fertility, 12(1 Pt 2), 77-126. PMID: 5419031

Trudnowski RJ, & Rico RC. (1974) Specific gravity of blood and plasma at 4 and 37 degrees C. Clinical chemistry, 20(5), 615-6. PMID: 4826961

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