Your period is called menstruation and it is part of your menstrual cycle. This cycle is a process where the woman’s body prepares itself for pregnancy. It is controlled by two main hormones, estrogen and progestin. These hormones will build up the lining of the uterus to prepare you for pregnancy and if you don’t become pregnant will start your period.
For most women, the average length of a cycle is 28 days but for some women it can be shorter or longer. Bleeding starts on the first day of your menstrual cycle. The reason why you bleed is to get rid of the thick lining of uterus from last month’s cycle. If you don’t get pregnant, the uterus doesn’t need all that extra stuff so it comes out as your period1.
Table 1: How do estrogen and progestin affect pregnancy and menstruation?
Ovaries release estrogen to make the lining of the uterus thicker
When a woman ovulates, an egg is released from an ovary, which results in progestin being released
Progestin makes changes to the thick uterine lining so that it is ready for pregnancy
If the egg is fertilized by sperm it becomes an embryo
This causes progestin levels to stay high which stops the lining of the uterus from breaking down
The embryo can then attach to the uterus, resulting in pregnancy
If the released egg is not fertilized then progestin levels go down
This causes the uterus to contract and the thick lining will come out, resulting in the bleeding from your period. Most women will bleed for an average of 5-7 days.1