Human reproductive systemWatch
The start of the cycle, day 1, is when bleeding from the vagina begins. This is caused by the loss of the lining of the uterus, with a little blood. This is called menstruation or having a period.
By the end of about day 5, the loss of blood stops. The lining of the uterus begins to re-grow and an egg cell starts to mature in one of the ovaries.
At about day 14, the mature egg cell is released from the ovary. This is called ovulation. The egg cell travels through the oviduct towards the uterus.
If the egg cell does not meet with a sperm cell in the oviduct, the lining of the uterus begins to break down and the cycle repeats.
The female sex hormones play a vital role in the above cycle. Oestradiol [most potent oestrogen] stimulates growth of the sex organs and maturation of the ovarian follicle, so its level rises before ovulation - this triggers the release of LH [luteinizing hormone] via gonadotrophin release from the pituitary, and LH stimulates ovulation. Once this is done, oestrogen levels drop greatly [its job is complete for this cycle].
The vagina is friendly to spermatozoa around the time of ovulation, for obvious reasons.