The menstrual cycle is the process by which the female body prepares for pregnancy. Since the average age of the first period is around 12.5 years, the association of periods with pregnancy can be (appropriately) alarming for most teens.
The changes that occur during the menstrual cycle are due to hormones released from the brain and ovaries that tell the uterus when to grow the uterine lining and then when to shed it.
The first day of the menstrual cycle is when the shedding occurs and when true period flow begins. The period flow of a normal menstrual cycle should not last longer than 7 days in adolescents. Once the shedding has finished, the hormone estrogen begins to increase which thickens the uterine lining. Around cycle day 14, the ovary releases an egg (ovulation), to see if it will get fertilized by a sperm to produce a pregnancy.
Once ovulation has occurred, the predominant hormone changes to progesterone which stabilizes and increases blood flow to the uterine lining. If pregnancy doesn’t occur, the progesterone level decreases and the lining sheds, starting the cycle over.
Because birth control pills contain mostly progesterone, they often lighten periods by limiting the growth of the uterine lining. Progesterone IUDs keep the lining thin as well, which lightens or eliminates periods during use. This is the same for Depo Provera. This is why hormonal medication can be therapeutic for period issues and concerns.
