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Have you ever wondered why we need to bleed every month? Each month, if your egg doesn’t get fertilised it comes away, along with your uterus lining. Usually at the most inconvenient time, right?

How menstruation works

Each month, cyclical changes take place in the ovary and uterus in preparation for pregnancy. At the beginning of each monthly cycle, the uterus lining is thin. It then builds up over the course of the month, and, if fertilisation doesn't occur, most of the thickness is shed in the form of a menstrual period.

Ovarian cycle
Uterine cycle

These changes are controlled by the hormones oestrogen, progesterone, follicle-stimulating hormone (FSH) and luteinising hormone (LH).
Each hormone has a different role within the cycle:

  • FSH; causes an egg to mature in an ovary and stimulates the release of oestrogen
  • Oestrogen; switches off the production of FSH so that only one egg is matured each cycle, and stimulates the release of LH
  • LH; causes the release of the mature egg from the ovary (ovulation)
  • Progesterone; maintains the lining of the uterus. When a period occurs the levels of progesterone are low

As you can see, when women are on the pill, these fluctuations are no longer present. This means there is no development of the egg, no ovulation and little build-up of the uterine lining.

menstrual cycles compared normal vs on the pill
So, without the cyclic fluctuations, why do women have a period when taking the contraceptive pill?

The “menstruation” that occurs whilst taking hormonal contraceptives is actually called withdrawal bleeding, and is caused by a change in the dosage of hormones in the contraceptive, e.g. during the placebo week of a 28-day pill, or the week each month when a patch user doesn’t use a patch, or during the low-dose week of a long-cycle pill. This change in dosage causes the lining of the uterus to weaken just enough for some bleeding to occur. This is usually lighter than a regular menstrual period.

Uterine lining

There is no medical need for withdrawal bleeding when taking a combined hormonal contraceptive as the uterus lining doesn’t build up in the way that it does when you are not taking a hormonal contraceptive.