Illustration of the uterus (right) and an ovary (centre left) on day nineteen of the menstrual cycle. Once sexual maturity is reached up to 20 oocytes start developing into eggs (ovums) every menstrual cycle. Only one will reach full maturity as a Graafian follicle and release its egg (pink, in fallopian tube) at ovulation. The follicle then develops into a corpus luteum (yellow, in ovary), which secretes progesterone to build up the endometrium, the bloody lining of the uterus (red), ready for a fertilised egg. If there is no fertilised egg after 12 days it stops secreting and degenerates into a corpus albicans. Without progesterone the endometrium sloughs off; this is menstruation. The cycle can now start again. Levels of female sex hormone at this stage are shown at bottom left. They are follicle stimulating hormone (FSH), luteinising hormone (LH), oestrogen (O) and progesterone (P).