The menstrual cycle is controlled by the female sex hormones. The entire cycle, from the beginning of one period to the beginning of the next takes about 28 days for most women and girls (can range between 25–35 days). The cycle starts on the first day of the monthly bleeding. During this period, blood and tissues that have built up on the inner lining of the uterus flow out of the vagina. This can last from two to seven days. The last day of the menstrual cycle is the day before the next monthly bleeding starts. For example, if bleeding started on January 1, that is the first day of the menstrual cycle; if the menstrual cycle is 28 days long, the next monthly bleeding would start on January 29. After menstruation, the girl’s body starts releasing hormones that signal the uterus to get ready to receive a new egg.
At about the same time, hormones prepare an egg to be released from the ovary. A woman’s egg is released once a month, usually about midway between two menstrual periods. The release of the egg from the ovary is called ovulation. It then travels down the fallopian tube where it stays for 24-48 hours and then travels to the uterus. Although the exact timing is difficult to predict, women with 28-day cycles are most likely to be fertile between days 8 and 15 of each cycle. This means that if a couple is not using family planning methods, vaginal intercourse can lead to pregnancy when it takes place during these days (or five days before the egg is released). The egg and sperm can reach the fallopian tube at the same time and an egg may be fertilized. The lining of the uterus thickens and becomes rich in blood and nutrients. The fertilized egg would then attach itself to this lining which provides a nourishing environment for the developing embryo.
If the egg is not fertilized by a sperm during this time, it will begin to dissolve. Most of the time the egg is not fertilized. This means that the uterus no longer needs the extra blood and tissue lining. A change in hormone level signals the blood vessels that nourish the lining to constrict and temporarily cut off the blood supply. This change in blood supply causes the lining to shed. The lining, blood and dissolved egg leave the body through the vagina as the next menstrual cycle begins. This is called menstruation and is also referred to as periods/menses/monthly period.