When the production of FSH increases, this results in maturation of an egg in the ovaries. The egg is located in an egg sac (follicle) which forms the female sex hormone oestrogen. Oestrogen builds up the endometrium.

Approximately 14 days before you expect your menstruation, the follicle is mature and now produces large quantities of oestrogen. This causes the pituitary gland to excrete large quantities of the sex hormone LH, which causes the follicle to burst and ovulation to occur. The remnants of the burst follicle (the yellow body) now produce the hormone progesterone, which helps to maintain pregnancy if the egg is fertilised.

If a sperm cell manages to fertilise the egg within approximately 24 hours, it will wander through the fallopian tubes down into the uterine cavity and attach itself to the endometrium after five days. If the woman becomes pregnant, she produces hCG (pregnancy hormone).
If the egg is not fertilised, it will be lost and there will be a decrease in the progesterone and oestrogen hormones, there is desquamation of the endometrium and menstruation will begin.

After ovulation, the egg can be fertilised for approximately 24 hours. Sperm can survive for approximately 48 hours after intercourse. Therefore, the optimal time for fertilisation starts two days before ovulation and ends one day after ovulation. I.e. approximately 14-16 days before the woman expects her menstrual period.