The menstrual cycle
The menstrual cycle is a cycle of events that occurs in the womb (uterus) and ovaries of female mammals. It is associated with the production of eggs and preparing the uterus for the implantation of fertilised eggs.
The menstrual cycle occurs over a period of about 28 days. The changes during the cycle are due to four hormones, progesterone, oestrogen, FSH (follicle stimulating hormone) and LH (luteinising hormone). Progesterone and oestrogen have wide ranging effects on the body but in the context of the menstrual cycle progesterone is mainly involved in maintaining the lining of the uterus and oestrogen is mainly concerned with building up the lining of the uterus. FSH stimulates the production of eggs and LH stimulates the release of the egg. FSH and LH are produced by the pituitary gland in the brain.
There are four stages in the menstrual cycle.
Stage 1. Days 1-4. Menstruation (bleeding) occurs. The lining of the uterus disintegrates and is shed. This is due to low levels of progesterone.
Stage 2. Days 4-14. The uterine lining grows back. This is due to high levels of oestrogen.
Stage 3. Day 14. The egg (called an ovum) is released. This is due to LH.
Stage 4. Days 14-28. The lining of the uterus is maintained in case the egg becomes fertilised and implanted in the uterus. Maintenance of the lining is due to high levels of progesterone.
The four hormones interact with each other. FSH causes Oestrogen release and oestrogen inhibits FSH. LH stimulates both oestrogen and progesterone production. Before ovulation LH release is stimulated by oestrogen but after ovulation it is inhibited by both oestrogen and progesterone.
In summary, the hormones have the following effects:
- Oestrogen: causes growth of the uterine lining. Inhibits FSH. Stimulates release of LH and hence release of the egg. Inhibits LH after ovulation.
- Progesterone: maintains the uterine lining. Inhibits LH after ovulation.
- LH: Stimulates the release of the egg (called ovulation). Stimulates oestrogen and progesterone production.
- FSH: Stimulates egg development and the release of oestrogen.
Birth control tablets contain high levels of progesterone and oestrogen. The oestrogen inhibits FSH production so that eggs cease to develop.
FSH is used to treat infertility because it stimulates the production of eggs.
Learning the menstrual cycle
The menstrual cycle is easy to learn. The best approach is probably to break it down into parts.
More advanced topics
This section deals with topics that are outside the GCSE syllabus.
At a more advanced level the menstrual cycle is divided into phases as shown in the diagram.