This phase starts on the first day of mensturation period . When ovum does not get fertilized , the high level of pogesterone hormone in the blood gets remarkably reduced .As a result , the endometrium layer of uterus gets degenerate and break down of capillaries . It causes a flow of blood from uterus for four days . This process of flow of blood along with the unfertilized egg is known as mensturation .
