At some point between the ages of 9-16 a girl will undergo puberty and start developing into a woman. Part of this development includes having their first menstrual period.

The menstrual period, also referred to as menstruation or menses, is just one part of the menstrual cycle. This cycle is 28 days long and occurs in all healthy women who are not yet in their menopause. The menstrual cycle is the body's way of preparing a woman for pregnancy.

Each month, the reproductive system prepares for the fertilization of a woman's egg and the development of an embryo. The process is depends on hormones, mainly estrogen, progesterone, follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

The menstrual cycle is made of three phases.
The first phase is menstruation. Each month as the uterus prepares for an embryo, it thickening the lining with tissue and blood. When an egg is released without being fertilized, the lining will be useless, so it will be shed out and passed out of the body through the vagina. The menstrual flow is composed of a mixture made up of blood, mucus, and body cells. The flow might be red or quite dark, and may include some clumps or clots. Periods typically last 2-7 days.

The second stage is the Follicular phase, also known as the proliferative phase. This is the stretch of time between the end of menstruation and ovulation. During this phase the uterus starts thickening its lining and preparing for the pregnancy.
At the end of this stage ovulation occurs and an egg is released from the ovaries into the fallopian tubes.

The luteal phase is the third stage, also known as the secretory phase. If the egg is not fertilized, the thickened lining of the uterus is expelled and a new menstrual cycle starts all over again.

A menstrual period may vary every month and differs from one woman to another. The blood flow could be light, moderate, or heavy and can be preceded by Pre Menstrual symptoms.

At around the age of 50, women reach menopause and stop having a menstrual period. This is because a woman is no longer ovulating and can no longer become pregnant. Like menstruation, menopause also varies from woman to woman and may even take a few years to occur.