Menstrual Phase

The cycle begins with the menstrual phase, during which menstrual flow occurs, typically lasting 3-5 days. This bleeding is the physical breakdown of the endometrial lining and its blood vessels, and it happens because progesterone and oestrogen levels have fallen sharply — the direct consequence of the corpus luteum degenerating after the previous cycle's released ovum went unfertilised. Because menstruation reflects the absence of a successful pregnancy, its sudden absence can be an early indicator that fertilisation and implantation have occurred; however, a missed period can equally be caused by u …