The menstrual phase (days 1-5): The reduction in progesterone after ovulation causes the shedding of the outer thick lining of the uterus (endometrium). The endometrium was richly supplied with blood vessels to nourish the fetus if a pregnancy occured so when it disintegrates and passes down the vagina, some blood is mixed with it. This is called menstrual flow, and occurs usually for a period of 3-5 days. 

The proliferative phase (days 6-14): The concentration of oestrogen in the blood is rising, following the end of menstruation, as the ovaries prepare for the next ovulation around day 14. The endometrium grows thicker in preparation of fertilisation and pregnancy.

The secretory phase (days 15-28): The blood concentration of progesterone increases, causing more blood vessels to grow into the endometrium. If the ovum is fertilised and the embryo implants in the endometrium and a placenta develops, it produces a hormone called human chorionic gonadotropin (HCG) throughout pregnancy.