The menstrual cycle
The menstrual cycle occurs from puberty until the end of reproductive life.
The purpose of the cycle is to prepare the female reproductive system for pregnancy.
It is controlled by the hormones oestrogen and progesterone.
A 28-day cycle:
- Menstruation occurs – the uterus lining breaks down and is released through the vagina.
- Oestrogen levels are low.
- Progesterone levels are low.
- The uterus lining begins to renew itself in preparation for the release of an egg.
- Oestrogen increases causing the initial repair of the uterus lining and its buildup.
- Oestrogen levels peak, this causes ovulation – the release of an egg.
- Progesterone levels peak after ovulation.
- Progesterone ensures the thickness of the uterus lining is maintained or the placenta and other pregnancy structures form if fertilisation has occurred.
- If pregnancy has not occurred the egg passes out of the vagina and the levels of both hormones drop, causing the cycle to begin again.
Sperm present in the female reproductive system in the few days before ovulation, could fertilise an egg when it is released.
Any sperm that enter the female reproductive system in the few days after ovulation could also fertilise an egg.