The usual interval between menstrual cycles generally ranges between 21 to 45 days, with the majority of women having an average cycle of 28 days. Menstrual periods can occur between two days to sometimes more than seven days, the average being 3 to 4 days in most women. It usually can be considered a short menstrual period when the menstrual bleeding lasts less than two days, or when the time interval between cycles is less than 21 days. The term eumenorrhea is technically used to denote a regular menstrual cycle.
Menstruation usually starts during puberty, at around the age of 12, although it can start as early as eight years old and as late as 16 years old. It generally is nature's way of preparing the body of a girl for pregnancy. The very first menstrual bleeding that occurs technically is termed menarche. During the first year after menarche, menstrual periods mostly are irregular. Some girls experience short menstrual periods, and at other times they may have longer menstrual periods.
The menstrual cycle is mostly influenced by the functions of hormones in the female body. A short menstrual period or a long menstrual period often depends on the amount of hormones present during each cycle. Menstrual cycles are divided into numerous phases: the menstrual phase, the follicular phase, ovulation, and the secretory phase or luteal phase. The menstrual phase starts on the first day of menstrual bleeding which often lasts for four days. Amount of blood loss is between 0.8 ounces (about 25 ml) to 2.5 ounces (about 75 ml) with an average of 1.35 ounces (40 ml) during each period.
After the last day of menstrual bleeding, the follicular phase usually follows. During the follicular phase, the ovary which contains approximately 450,000 follicles at birth, is influenced by the follicular stimulating hormone (FSH) to ripen some eggs. At the same time the female hormone estrogen also rises and technically influences the lining of the uterus, or endometrium, to initiate the thickening process in preparation for pregnancy.
At approximately the fourteenth day, counting from the first day of menstruation, ovulation occurs. This mostly signals the release of a mature egg from the ovary with the influence of a hormone called luteinizing hormone (LH). Another hormone, called progesterone, also rises to make the endometrium technically ready for the implantation of a fertilized egg. When the egg is fertilized by the sperm, pregnancy occurs and no menstruation generally happens during the next nine months.
The secretory phase often follows the ovulation period when no fertilization takes place, lasting approximately 10 to 16 days. This largely is influenced by the decline of the hormone progesterone. At the end of the secretory phase, the endometrium sheds off to start the first day of menstruation and of another menstrual cycle.
A woman's reproductive years mostly last until about the age of 45. The complete cessation of menstruation is commonly called menopause. Many changes often occur before menopause. Some women experience irregular cycles, sometimes having long or short menstrual period, and sometimes with light and heavy flow.