The female menstrual cycle

With the increased production of FSH, maturation of eggs in the ovaries begins. Each of these eggs is located in a follicle, which is responsible for the production of the female sex hormone (oestrogen). This hormone builds up the lining of the uterus (endometrium) and opens the cervix slightly. It also promotes the production of cervical secretions, which make it easier for sperm to enter the uterine cavity. At some point, the body chooses the largest follicle to continue growing and stops the growth of the others. The largest follicle is called the 'dominant' follicle. This ensures that only one egg will ovulate. However, in some women, two eggs may mature in the natural cycle. This happens more frequently the older the woman. Around 1000 eggs compete to become the dominant follicle each month.  

In the middle of the menstrual cycle, the egg in the follicle has fully matured and is producing maximum amounts of oestrogen. Due to the increased amount of oestrogen, the pituitary secretes larger amounts of LH. This sex hormone causes the follicle to rupture, allowing the egg to emerge from the ovary, and this is known as ovulation. The remains of the follicle form the corpus luteum, which produces the hormone progesterone. In the event of fertilisation, progesterone helps to maintain pregnancy. The mature eggs are captured by the fallopian tube and transported towards the uterus. The eggs are viable and can be fertilised in about 24 hours.   

The egg can be fertilised while it is in the fallopian tube. If a sperm reaches the egg and succeeds in penetrating the embryonic membrane, the egg divides several times before it reaches the uterus. After a further four to five days, the fertilised egg attaches itself to the lining of the uterus. This immediately starts to release the signalling hormone hCG (human chorionic gonadotropin, also known as the pregnancy hormone) to maintain the pregnancy.  

If the egg in the fallopian tube is not fertilised, this is detected by the ovary, because the uterine hCG signals are absent. The yellow body formed by the follicular debris perishes after 10-14 days. This means that the progesterone level in the woman's blood falls, and the uterine lining is shed. This marks the beginning of the first day of the new menstrual cycle.