Producing fertile eggs (oocytes)
Women are born with about 400,000 immature eggs already in their ovaries. Each month, between puberty and menopause, one egg (usually) fully matures and starts its journey down the fallopian tubes in the anticipation of fertilisation.
There are three stages to a woman's "monthly" menstrual cycle. On average, this cycle lasts 28 days.
The first stage of the cycle lasts for about two weeks. During this stage, the secretion of FSH rises, stimulating the development of an egg-containing follicle and the maturation of the egg within it. The growing follicle secretes increasing amounts of the female hormone, oestrogen, which triggers changes in the lining of uterus (endometrium) and cervical mucus. The cervical mucus thins to allow sperm to pass through and the endometrium thickens making it ideal for the implantation of a fertilised egg.
About 32 hours before an egg is ready to be released, the amount of oestrogen produced by the follicle sharply increases, causing a spike in the secretion of LH by the pituitary gland. This surge in LH production causes ovulation. The matured egg bursts out of the follicle and travels down the fallopian tube.
The remains of the follicle become a corpus luteum, which secretes a second female hormone, progesterone. This helps maintain the best conditions for pregnancy should the egg be fertilised.
If the egg is not fertilised within about 72 hours, the corpus luteum eventually degenerates and the egg is expelled from the uterus along with the lining, leading to menstruation approximately 14 days later.