The menstrual cycle has four stages
- Stage 1: Day 1 is when the bleeding starts. The uterus lining breaks down and is released.
- Stage 2: The lining of the uterus builds up again, from day 4 to day 14, into a thick spongy layer of blood vessels ready to receive the fertilised egg.
- Stage 3: An egg is released from the ovary (ovulation) at about day 14.
Stage 4: The lining is then maintained for 14 days, until day 28. If no fertilised egg has landed on the uterus wall by day 28 then the spongy lining starts to break down again and the whole cycle starts over.
Hormones and the menstrual cycle
- FSH (follicle-stimulating hormone): This causes a follicle (an egg and its surrounding cells) to mature in one of the ovaries. It stimulates oestrogen production.
- Oestrogen: This causes the uterus lining to thicken and grow. A high level stimulates an LH surge.
- LH (luteinising hormone): LH stimulates ovulation at day 14 – the follicle ruptures and the egg is released. The remains of the follicle is also stimulated to develop into a structure called a corpus luteum which secrets progesterone.
- Progesterone: This maintains the lining of the uterus. It inhibits the production of FSH and LH. When progesterone levels fall, and there is a low oestrogen level, the uterus lining breaks down. A low progesterone level allows FSH to increase and the whole cycle starts again.
- If the egg is fertilised and is implanted in the uterus (pregnancy) the level of progesterone will stay high to continue to maintain the uterus lining.
- The uterus lining has a thick spongy layer of blood vessels – this blood supply allows the placenta to develop. The placenta delivers the baby with oxygen, glucose and nutrients it needs to grow and removes its waste products (urea and carbon dioxide).
Negative feedback controls levels of hormones
- The different hormones in the blood during the menstrual cycle are controlled by negative feedback. FSH is controlled by negative feedback for example:
- FSH stimulates the ovary to release oestrogen.
- Oestrogen inhibits further release of FSH from the pituitary gland.
- After FSH has caused a follicle to mature, negative feedback keeps FSH levels low. This ensures no more follicles mature.
