Sex hormones in girls remain dormant until puberty when the hypothalamus, a small gland at the base of the brain, makes a master hormone. This instructs the pituitary gland to release into the blood two powerful hormones, follicle stimulating hormone (FSH) and lutenising hormone (LH). These two hormones are responsible for the development and release of an egg from the ovary.
 
At the beginning of the menstrual cycle the levels of oestrogen and progesterone are very low as a result of the shedding of the specially prepared womb lining. This happens when the fully mature egg is not fertislised by the sperm.
 
The hypothalamus senses these low levels and causes the release of FSH (follicle stimulating hormone). The function of this is to ripen an egg in an ovary for release and fertilisation. It also stimulates the production of oestrogen by the ovary. This level rises over the first half of the cycle and peaks at about Day 12.This signals the hypothalamus to release LH (lutenising hormone). Around Day 14 a surge of LH brings about ovulation. Ovulation to menstruation takes part in the second half of the cycle – days 14 – 28. The space left in the ovary after the egg has been released fills with blood and special cells and builds into a dense mass, the corpus luteum. This now becomes the manufacturing site for oestrogen and progesterone in the second half of the cycle. High levels of both of these hormones are required to support fertilisation should it occur.
 
If the egg is not fertilised the corpus luteum breaks down which causes a rapid fall in the levels of both oestrogen and progesterone. This low level acts to stimulate the hypothalamus to release FSH and LH and the process starts all over again.