Menstruation is commonly called  menses, a period or menstrual bleeding. Menstruation is the approximately monthly shedding of the lining of the uterus.

Menstrual blood flows from the uterus through the small opening in the cervix, and passes out of the body through the vagina The length of bleeding also varies from one to eight days, with the average length being five days. A girl start to menstruate at any time between the ages of 8 and 16. Commonly, the first period starts about two years after breasts first start to grow continues until menopause. Menopause occurs between the ages of 45 and 55, usually around age 50. Menopause means that a woman is no longer ovulating (producing eggs) and therefore can no longer become pregnant. Like menstruation, menopause can vary from woman to woman and may take several years to occur. Some women have early menopause because of surgery or other treatment, illness, or other reasons. The time when your body begins its move into menopause is called the menopausal transition. This can last anywhere from two to eight years.

The Duration of the Period Menstuation:

Menstruation is part of the menstrual cycle.  A cycle starts on the first day of a period. The duration of the period varies between women. The average menstrual cycle is 28 days long.  However, a cycle can range anywhere from 23 days to 35 days. It usually last for 3-7 days during each month. Any bleeding which is less or more than this duration may cause irregular menstruation.

The Menstrual Cycle
The start of menstruation is one of many signs that a girl has become a young woman. It means that it is now possible for a girl to get pregnant and have a baby.  As you mature, your brain signals your body to produce hormones. These hormones serve as messengers to trigger your menstrual cycle. A part of that cycle is the monthly period.

Each month, after day 5 of your cycle, levels of estrogen rise and the lining of the uterus (endometrium) begins to grow and thicken. If a woman were to become pregnant, that extra-thick lining would help to nourish the developing fetus (a baby growing in the woman’s uterus).

Around cycle day 14, an egg is released from one of your ovaries. This is called ovulation. the lining of the uterus (endometrium) begins to grow and thicken. If a woman were to become pregnant, that extra-thick lining would help to nourish the developing fetus (a baby growing in the woman’s uterus). The egg moves into one of the two fallopian tubes connected to the uterus. If the egg is fertilized by a man’s sperm, it moves into the uterus, where it attaches to the lining, and begins to grow into a fetus.

Around cycle day 28, if an egg is not fertilized, the endometrium is shed by bleeding. The menstrual blood (a mix of blood and tissue from the inside of the uterus) flows from the uterus through the cervix into the vagina and out of the body. This bleeding is your menstrual period. Your period may last for 3-5 days. Some last as long as 7 days. The process then starts again in the next cycle.

Hormonal
various types of hormones are involved:

Estrogen:
1) Promotes the development and maintenance of female reproductive structures (especially the endometrial lining of the uterus).
2) Assists in the control of fluid and electrolyte balance within the body.
3) Prepares the follicle for the release of an egg.
Estrogen also has many other functions.

Progesterone:
Secreted at ovulation, helps to prepare the endometrium (womb lining) for the implantation of an egg, prepares mammary galnds for milk production. Primarily concerned with the procreation and survival of the fetus.

FSH ( Follicle Stimulating Hormone):
Stimulates the follicles (a follicle is a balloon shaped structure which is filled with fluid and contains an egg, follicles are found in the ovaries) to ripen several eggs. At the same time the ovaries release oestrogen.

LH  (Lutenising hormone):
Further develops the follicles, triggers ovualtion and stimulates production of other hormones necessary for the post ovulatory stage of the menstrual cycle.
The secretion of hormones is a complex affair. Various parts of the body become involved in a myriad of chemical transactions.

Finally
Menstruation is a normal, healthy process. In most cases, it does not affect your day-to-day activities. If you have any concerns about your menstrual cycle, discuss them with a trusted adult or your doctor.

• Abnormal Vaginal Bleeding
• Can You Have Your Period and Still be Pregnant?
• Submucosal Fibroids
• Best Way to Get Pregnant: 5 Important Facts About Getting Pregnant You Must Know.
• Do You Suffer from Irregular Menstruation?