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Menstruation, the periodic shedding of the uterine lining (the endometrium) accompanied by bleeding, is a natural process that occurs in pre-menopausal women who are not pregnant or breast-feeding. Menstruation marks the fertile years of a woman’s life when she is able to bear children. Menstrual cycles (menses) typically begin in girls two to three years after the onset of puberty (around age 12 or 13) and continue until menopause (around age 48 to 52).

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The Female Reproductive System

The female reproductive system is made up of both external and internal organs.

Vulva: The term vulva refers to the external parts of the female genitalia. The upper portion of the vulva is covered with pubic hair in mature females. The lower portion of the vulva consists of the labia majora (outer lips), which are two folds of fatty tissue covered with skin. Inside the labia majora are the labia minora (inner lips), another two folds of tissue. The labia minora protect the opening of the urethra (urine canal) and the entrance to the vagina. Above the urethra, the labia minora join to form the tip of the clitoris. The clitoris plays a key role in female sexual arousal.

Vagina: The vagina is the canal that leads from the uterus to the outside of the body. It serves as a passageway for menstrual discharge and as the birth canal. In young girls, the opening of the vagina is covered by a thin membrane called the hymen. As a woman begins to increase physical activity, use tampons, or engage in sexual intercourse, the hymen is gradually stretched.

Cervix: The cervix is the lower portion of the uterus that connects the uterus to the vagina. The opening of the cervix remains small except during labor when it expands to allow the baby to pass from the uterus to the vagina. Click here to learn more about cervical cancer.

Uterus: The uterus is a pear-shaped muscular sac that houses the fetus during pregnancy. During each menstrual cycle, the lining of the uterus (endometrium) thickens to prepare for possible pregnancy. If a woman does not become pregnant during this time, the endometrium is shed and exits the body through the vagina each month during menstruation.

Fallopian tubes: The Fallopian tubes are attached to the uterus and extend toward the ovaries. At the ends of each tube are fringe-like structures called fimbriae that wrap around each ovary. These fimbriae help guide the reproductive eggs (ova) that are released by the ovaries each month into the Fallopian tubes. When an egg enters one of the Fallopian tubes, a series of contractions force the egg down toward the uterus. While an egg is traveling from the ovaries to the uterus through one of the Fallopian tubes, it may be fertilized by sperm. Sperm are released into the female vagina during sexual intercourse.

Ovaries: The ovaries are located on both sides of uterus. In addition to producing the hormones estrogen and progesterone, the ovaries also produce mature reproductive eggs (ova) that are released each month during ovulation in pre-menopausal women. If an egg becomes fertilized by sperm while it travels from the ovaries to the uterus through one of the Fallopian tubes, then it implants itself in the uterus where it will grow and evolve into a mature baby over the next nine months. If the egg is not fertilized by sperm, then it breaks down along with the uterine lining and is discarded during menstruation.

Normal Female Reproductive Development

8 weeks in utero Around 8 weeks after conception, it is possible to distinguish a female fetus from a male fetus. The female reproductive organs develop before birth.
Age 6 As early as age 6, there are noticeable physical differences between girls and boys. In girls, the buttocks tend to be rounder, the shoulders are narrower, and the hips are wider.
Age 12 Age 12 is the average age of menarche (the first menstrual period). At this age, a girl is about halfway through puberty, the time when the breasts grow, the hips widen, and pubic hair appears. The onset of menstruation marks the beginning of the reproductive years. The menstrual periods may be irregular for the first 1-2 years.
Age 18 By age 18, most young women have reached their adult height, weight, and shape. Menstrual periods become more regular in most women by this time.
Ages 44-48 As a woman approaches menopause, her body produces less estrogen and progesterone. This depletion in hormone levels can cause irregular menstrual cycles, hot flashes, vaginal dryness, and other menopausal symptoms. The length of this phase (called perimenopause) and the associated symptoms varies significantly from woman to woman.
Age 50 Between the ages of 48 and 52, most women reach menopause and are no longer able to bear children. After menopause, the vagina may grow smaller and less elastic, and the uterus and ovaries decrease in size. Typically, menopausal symptoms, such as hot flashes, decrease a few years after menopause.

Onset of Menstruation

The onset of menstruation (menarche) typically begins around age 12 or 13. However, some girls will begin menstruating a few years earlier or later. Menstruation usually begins approximately two or three years after the first physical signs of puberty appear (such as breast buds or pubic hair). Girls who do not begin menstruating by age 16 should be examined by a physician, although in the majority of cases, there is no cause for concern. Early menstruation (before age 12), late menopause (after age 50), never having children, or having a first child after age 30 can increase the risk of breast cancer

The menstrual cycle is often irregular for the first one to two years. During this time, it is not uncommon for young girls to go three or four months without menstruating. However, over time, the cycle becomes more regular and occurs every 28 days on average (although some women will have shorter or longer cycles).

Recent studies show that girls are beginning to menstruate at an earlier age than previous generations. Researchers are investigating a variety of factors that might influence early menstruation including obesity, environmental estrogens found in chemicals and pesticides, and nutrition.

Length of Menstruation and the Menstrual Cycle

The length of a menstrual cycle varies significantly from woman to woman. The average length is 28 days, although some women have shorter or longer cycles, with a range of approximately 24 to 35 days in 95% of women. The average length of a menstrual period is approximately five days. Again, some women experience shorter or longer periods, with a range of approximately one to eight days. The average blood lost during a period is only two ounces (60 milliliters).

Factors that may change the length of menstruation or menstrual cycles include:

  • Intense exercise
  • Dramatic weight loss
  • Smoking (smokers, especially thin women who smoke, tend to have longer periods)
  • Stress
  • Travel
  • Change in contraception methods
  • Unsuspected pregnancy
  • Early miscarriage
  • Recent childbirth
  • Peri-menopause (the years immediately before menopause)
  • Disorders of the uterus (such as uterine fibroids or endometriosis; see menstrual problems section below)

In addition, studies have shown that when several young women live together and do not take oral contraceptives, they tend to menstruate at the same time of the month. Researchers attribute this synchronization of menstrual cycles to the secretion of pheromones, chemicals that may influence biological activity. Synchronization is disrupted if women live with men.

The Menstrual Cycle

A woman’s menstrual cycle begins each month when the pituitary gland secretes a follicle-stimulating hormone (FSH). This occurs once every 28 days on the first day of bleeding. FSH stimulates a few of the hundreds of reproductive ova (eggs) in one of the ovaries to begin to mature. As an egg matures, the cells around it also grow and form a follicle that releases estrogen. Estrogen is a hormone that stimulates the growth of the uterine lining (the endometrium) to prepare the body for possible pregnancy.

The levels of estrogen and FSH increase for approximately 14 days until the pituitary gland secretes another hormone called luteinizing hormone (LH). LH causes the most mature egg in the ovary to be released around day 14 of the cycle. This act of releasing an egg is called ovulation. Sometimes, more than one egg is released, which can result in twins, triplets, etc. if all of the eggs are fertilized by sperm. When the egg is released from the ovaries, it is guided down one of the Fallopian tubes leading into the uterus.

During this time, the follicle that produces the mature egg (called the corpus luteum) in the ovary will grow and secrete progesterone, a hormone that further prepares the uterus for possible pregnancy. A woman is most fertile during this time, approximately two weeks before she menstruates. If the egg is fertilized during this time, then the woman becomes pregnant. If the egg is not fertilized by a sperm, then it breaks down along with the uterine lining and is discarded during menstruation. Progesterone and estrogen levels also decrease and the corpus luteum is absorbed by the body at this time. The cycle is then repeated.

Phases of the Menstrual Cycle

Follicular Phase Luteal Phase
  • Days 1-14
  • Pre-ovulatory, before the mature egg is released from the ovary
  • Days 15-28
  • Post-ovulatory, after the mature egg is released from the ovary

Physical and Emotional Changes Associated with the Menstrual Cycle

The majority of women experience some physical or emotional symptoms associated with their menstrual cycles (premenstrual syndrome, PMS). Common symptoms may include breast tenderness, bloating, temporary weight gain, irritability, anxiety, etc. Approximately 75% of women experience mild or moderate symptoms as they approach menstruation while a small percentage experience severe symptoms. The types of symptoms vary significantly from one woman to another and may change over time.

Examples of Premenstrual Symptoms

  • Breast tenderness
  • Bloating
  • Temporary weight gain
  • Headache
  • Backache
  • Fatigue
  • Cramps
  • Intolerance of alcohol
  • Sensitivity to light
  • Acne
  • Diarrhea
  • Throbbing varicose veins
  • Vaginal itching
  • Difficulty concentrating
  • Sleeplessness
  • Food cravings
  • Anxiety
  • Depression
  • Irritability
  • Mood swings
  • Muscle or joint pain
  • Hot flashes


Many premenstrual symptoms can be relieved or improved with lifestyle modifications and over-the-counter medications such as aspirin, ibuprofen, and naproxen sodium. For example, making changes to diet approximately 14 days before a period can help reduce symptoms. Dietary changes may include reducing red meats and dairy products; eating small, more frequent meals; avoiding high fat foods; reducing salt, sugar, and caffeine intake, etc. Vitamin and calcium supplements and exercise may also help alleviate premenstrual symptoms. In some cases, medications such as oral contraceptives, other hormonal therapies, or antidepressants may be necessary.

Approximately 3% to 8% of women experience premenstrual dysphoric disorder (PMDD, also called late-luteal dysphoric disorder). This condition should be evaluated by a physician. Women with PMDD experience severe irritability, depression, and several other symptoms including at least five of the following:

  • Feelings of sadness or hopelessness, possible suicidal thoughts
  • Feelings of tension or anxiety
  • Mood swings marked by periods of crying
  • Persistent irritability or anger that affects other people
  • Disinterest in daily activities and relationships
  • Trouble concentrating
  • Fatigue or low energy
  • Food cravings or bingeing
  • Sleep disturbances
  • Feeling out of control
  • Physical symptoms, such as bloating, breast tenderness, headaches, and joint or muscle pain

Source: The American Psychiatric Association

Though research is ongoing, it is believed that PMDD occurs from a chemical imbalance of serotonin in the body. In some cases, drug therapy such as Sarafem (generic name, fluoxetine hydrochloride) may be helpful. Women who experience severe premenstrual symptoms should discuss treatment options with their physicians.

Menstrual Problems

The following are examples of possible menstrual problems. While most menstrual abnormalities are not cause for concern, they can sometimes signal other medical problems. Therefore, all persistent menstrual abnormalities should be evaluated by a physician. Many problems can be effectively treated with hormonal therapies or other options.

Dysmenorrhea (severe cramps and pain): Uterine contractions are normal during the menstrual period. However, for some women, these contractions cause severe lower abdominal cramps that can begin several hours or days before the onset of menstruation. Dysmenorrhea is divided into two categories: primary and secondary. Primary dysmenorrhea is characterized by intense menstrual cramps that usually begin within the first three years of menstruation. Primary dysmenorrhea is caused by normal uterine contractions. More than 50% of all women suffer from primary dysmenorrhea. Many women find that nonprescription drugs such as aspirin, ibuprofen, and naproxen sodium relieve these cramps. Secondary dysmenorrhea is the onset of severe cramps that occur from other medical conditions, such as non-cancerous uterine tumors (fibroids) or endometriosis (a condition in which the uterine tissue lining grows outside of the uterus).

Menorrhagia (heavy menstrual periods): Most women who need to change tampons or pads frequently or experience long periods (more than six days) do not have dangerously heavy periods (menorrhagia). In many cases, these women may simply have thicker uterine linings that cause them to bleed longer. However, heavy bleeding can be a sign of other conditions, such as non-cancerous uterine tumors (fibroids), pelvic inflammatory disease, abnormal blood clotting, or other problems. Women who experience heavy periods should report the problem to their physicians for further investigation. Heavy bleeding that is independent of other conditions can often be treated with oral contraceptives or other hormonal therapy.

Metrorrhagia (bleeding between periods): Vaginal bleeding that occurs between periods (metorrhagia) may sometimes be called breakthrough bleeding or "spotting." There are several causes of metorrhagia, including oral contraceptives (usually only within the first one to three months of use), a dislodged intrauterine device (IUD), infection, inflammation of the uterus, polyps on the cervix, or an early miscarriage. Bleeding between periods is also common in young women when they just begin menstruating or near the time of ovulation (approximately 14 days before menstruation) in young adult women.

Oligomenorrhea (irregular or abnormally light menstrual periods): Oligomenorrhea is defined as having fewer than eleven menstrual periods a year. This is common in young women who just begin menstruating and women who are approaching menopause. Other causes of oligomenorrhea including pregnancy, thyroid disease, an acute illness, the use of oral contraceptives, stress, emotional problems, etc. Persistent, unexplained irregular periods should be evaluated by a physician.

Primary amenorrhea (failure to begin menstruating): The most common cause of primary amenorrhea is late puberty, which is usually genetically inherited and is not a cause for concern. However, if menstruation has not begun by age 16, clinical investigation is usually warranted. Occasionally, primary amenorrhea may be caused by hormonal problems or abnormalities of the pituitary gland, thyroid or adrenal glands, or the ovaries. Sometimes, hormonal therapy can help treat primary amenorrhea. In rare cases, failure to begin menstruating signals a missing part of the reproductive tract (such as the ovaries, uterus, or chromosomal abnormality).

Secondary amenorrhea (absence of menstruation): The secondary form of amenorrhea occurs after a regular menstrual cycle has already been established. The absence of menstruation for four months or longer is considered to be amenorrhea. Secondary amenorrhea has several causes including pregnancy, breast-feeding, discontinued use of oral contraceptives, too little body fat, drastic dieting or eating disorders (such as anorexia), intense exercise, stress, chronic diseases (such as thyroid disorders), or the use of tranquilizers or antidepressants. In many cases, menstruation returns on its own and there is little cause for concern unless the woman wants to become pregnant. However, women who do not menstruate for four months or longer should be evaluated by a physician to determine whether lifestyle changes or medications (such as oral contraceptives or other hormonal therapy) are necessary.

Resources for Information on Feminine Products

The following Web sites are maintained by companies that sell feminine products. They include information on menstruation, tampons, pads, and related products and issues. This list of Web sites is provided merely as a convenience to users. Imaginis does not endorse and takes no responsibility whatsoever for the information found on the following websites or for any aspect of the clinical trials listed therein.


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Additional Resources and References

Updated: August 17, 2007