Health Conditions

Puberty - ah, the wondrous changes girls must endure to become women! Menarche (the start of menstrual periods) is just one of those changes that occur during the early teen years (ranging from ages 9 to 16, but usually between the ages of 12 and 13). Irregular menstruation may be perfectly normal in the first year or two after menarche, but the cycles should become more consistent ? occurring approximately every 28 days and lasting between 3 and 7 days, with 4 or 5 days the most common length. Menarche usually means that a woman has started her fertile (child-bearing) years.

Menstruation (the "period") is just a part of the monthly cycle for the majority of women. Although pregnancy can start at any time, human fertilization usually takes place only at particular times within a monthly cycle.

The menstrual cycle can be divided into three phases that involve the ovaries:

Preovulatory Phase (also known as Follicular Phase)

Ovulatory Phase

Postovulatory Phase (also known as Luteal Phase)

Alternately, the menstrual cycle may also be divided into three phases that focus on the uterus:

Menstrual Phase

Proliferative Phase

Secretory Phase

The preovulatory phase occurs in the ovaries at the same time as the menstrual and proliferative phases occur in the uterus. Beginning both is the menstrual period. The first day of the period is also the first day (day one) of the cycle. During menstruation, blood levels of the hormone estrogen are low, causing the brain to activate the release of other hormones. These hormones, mostly follicle-stimulating hormone (FSH) and luteinizing hormone (LH), stimulate the maturing of follicles (egg sacs) in the ovaries. As the follicles mature, they start producing estrogen. The menstrual period ends and increasing estrogen levels cause fluids to begin accumulating in the endometrium (the lining of the uterus).

Near the end of the preovulatory/proliferative phase, estrogen levels reach a peak that, in turn, causes a surge of LH and the ovulatory (egg-release) phase begins. A sudden increase of LH causes final growth and maturing of the follicle. Ovulation occurs when an egg is released from the fully-matured follicle, usually on about day 14 of a 28-day menstrual cycle. Several follicles mature in any given cycle, but usually only one releases an egg, which enters the fallopian tube. It travels through the fallopian tube into the uterus. A follicle that has released an egg is called a corpus luteum.

The final phases ? postovulatory and secretory ? occur at the same time. After the release of the egg, the corpus luteum produces estrogen and large amounts of another hormone, progesterone. Glands in the uterus secrete a thick fluid in response to progesterone. Cells in the endometrium soak up the fluid and blood to become a thick layer that can trap and hold a fertilized egg.

Fertilization, also called conception (the joining of a male?s sperm with a female?s egg to create a new individual), takes place in the fallopian tube. The 12 hours to 48 hours that the egg survives is the time most likely for fertilization to take place; although a woman can become pregnant at other times during her cycle. Only one sperm is needed to fertilize an egg, but millions are released at a time. Some of the sperm may survive inside a woman?s body for as long as a week. If an egg that has been fertilized becomes trapped (implanted) in the thickened endometrium, a hormone known as human chorionic gonadotropin (hCG) is released. hCG causes the corpus luteum and the follicles to continue producing estrogen and progesterone throughout the resulting pregnancy. Therefore, estrogen and progesterone levels do not fall and menstruation usually does not occur during pregnancy. Generally, hCG is measured for pregnancy tests because it appears in the blood and urine of a pregnant woman as early as 6 days after fertilization.

An unfertilized egg, however, is not trapped by the thick folds of the endometrium. If implantation does not occur by approximately the 25th day of the cycle, both the egg and the corpus luteum disappear gradually. No hCG is produced, estrogen and progesterone production declines, and the fluid in the endometrial lining passes out of the vagina as menstrual blood - beginning the cycle again. Menstrual cycles may be disrupted by pregnancy, illness or other causes; but typically, they continue until a woman reaches menopause (the end of menstruation and fertility) in her late forties or early fifties.

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Note: The above information is intended to supplement, not substitute for, the expertise and judgment of your physician, pharmacist, or other healthcare professional. It is not intended to diagnose a health condition, but it can be used as a guide to help you decide if you should seek professional treatment or to help you learn more about your condition once it has been diagnosed.