Menstruation is the discharge of blood and tissue that occurs each month as part of a woman’s menstrual cycle. This cycle is controlled by hormones produced in both the brain and ovaries and prepares the reproductive organs for pregnancy. The lining of the uterus (endometrium) thickens in response to the hormones. If pregnancy does not occur, the lining begins to break down and discharges from the body through the vagina as the menstrual period.

Menstruation usually begins when a girl is about 12 or 13 years old (puberty) and continues until menopause. These are considered a woman’s reproductive years and indicate the woman is physically able to become pregnant. Menstrual periods usually occur once a month and last for several days during each month. A missed menstrual period is frequently the first sign that pregnancy has occurred. The uterine lining is not shed, but rather will grow during pregnancy and develop into the placenta (a temporary organ that provides nourishment for the developing embryo). Missed periods may also indicate the presence of other gynecological conditions, such as hormonal or endocrine disorders.

The blood flow of menstruation may vary from month to month. Women typically use sanitary napkins (pads) or tampons to absorb menstrual flow. Before and during menstruation, women may experience some mild to moderate cramps, water retention and irritability. Some of these symptoms may be treated with over-the-counter analgesic (painkiller) medication. However, symptoms that do not respond to painkillers or that worsen over time may indicate more serious conditions, such as endometriosis.

Hormones control the functions of the menstrual cycle. They signal the ovaries to develop and release the eggs. Hormones also signal the endometrium to initially begin to grow and develop, and to break down when menstruation occurs. As a woman ages and her hormone levels begin to decrease, the menstrual cycle eventually ends (menopause). Menopause typically occurs during a woman’s early 50s.

Menstruation is the monthly shedding of tissue and blood that is the most visible part of a woman’s menstrual cycle. The menstrual cycle involves all of the woman’s reproductive organs. It is an indicator of good health for reproductive-aged women and also prepares the body for pregnancy.

The uterus is a fist-sized, hollow organ in the lower abdomen that can expand to accommodate a growing fetus. The bottom of the uterus (cervix) meets the top portion of the vagina, the tube that leads outside the body. Menstrual blood leaves the body through the vagina. The lining of the uterus (endometrium) includes blood vessels, glands and tissue that grow and thicken to support a fetus during pregnancy. When pregnancy does not occur, the endometrium breaks down and is shed each month during menstruation.

A woman has a fallopian tube and ovary on either side of the uterus. Each fallopian tube extends from the top of the uterus to an ovary, the organs that produce the woman’s eggs. Every month, a mature egg leaves one of the ovaries and travels through the fallopian tube to the uterus. If the egg is fertilized by a sperm, it can implant in the endometrium of the uterus and develop into an embryo. If the egg is not fertilized, it disintegrates and is absorbed into tissue or leaves the body with the menstrual flow.

The first menstrual period (menarche) is part of the sexual development girls go through during puberty. Many girls get their first period around the age of 12 or 13 years. However, the age of onset varies considerably and girls may begin menstruating any time from age 8 to 16.

Both the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists recommend that adolescent girls see their primary care physician for preventive healthcare visits to discuss their reproductive health, and help establish an environment where they can feel comfortable discussing health concerns. Preventive health visits during adolescence can offer an opportunity for physicians to educate teen girls and their parents on what comprises normal puberty, menarche and the menstrual cycle.

During the first few years of menstruation, a girl’s period may be irregular. Eventually it should develop into a more regular monthly cycle. The average menstrual cycle lasts about 28 days, but may range from 21 to 35 days. Normal cycle length varies among women and for a particular woman at different times in life.

The menstrual period usually lasts about five days each month for most women. However, it can last from two to eight days. The flow of menstrual blood may be heavy for several days and then light. It may vary in volume and length from month to month. The use of birth control pills may make menstrual periods lighter and shorter.

Women use various products to absorb menstrual flow. Sanitary napkins (pads) are worn inside the panties. Tampons are inserted into the vagina. Both are made of cotton or another absorbent material. Pads or tampons should be changed frequently (every few hours).

Menstruation continues monthly for most women in their reproductive years. Pregnancy interrupts menstruation because the endometrium must nourish the growing fetus. Menstruation resumes after childbirth, sometimes taking several months to return to a normal cycle. Breastfeeding may also affect the return of menstruation.

By about age 50, women’s estrogen levels begin to decrease. The ovaries no longer ovulate (produce eggs) regularly and eventually women stop menstruating. After one year without a period, a woman is considered to have entered menopause. A woman in menopause cannot become pregnant. Surgical removal of the ovaries also causes a woman to reach menopause immediately, regardless of age. Many women experience several years of transition to menopause (perimenopause), when the menstrual periods may become inconsistent.

The menstrual cycle is a result of the complex interaction of several hormones. Many hormones are involved, but the four major ones are follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen and progesterone. The interaction of these hormones generally affects two areas of the reproductive system the most: the uterus and the egg follicles in the ovaries.

The menstrual cycle can be divided into two major phases:

• The follicular phase. This phase includes the first half of the menstrual cycle, beginning with the menstrual period and ending with ovulation. In a 28-day cycle, this includes the first 14 days. The first day of the menstrual period is counted as day one of the cycle. The uterine lining (endometrium) is shed during the menstrual period for about the first five days of the follicular phase.  
  
  Two hormones are dominant during this follicular or proliferative phase:
  
  
• Follicle stimulating hormone (FSH). Produced in the pituitary gland in the brain. FSH stimulates about 15 to 20 egg follicles to begin developing in one ovary. FSH stimulates the follicle development and also stimulates the follicles to produce estrogen.
  
  
• Estrogen. Produced by the growing follicles. As estrogen reaches a certain level, it causes the endometrial lining (which was just shed as menstrual blood) to begin to grow again. As the estrogen level rises, it eventually triggers a stop in FSH production, which prevents more follicles from developing.
  

During the follicular phase, one developing egg follicle becomes larger than the others (dominant follicle). At about day 14, the estrogen from this follicle triggers a surge and release of LH from the pituitary gland, initiating ovulation. The egg follicle bursts and releases the mature egg into the fallopian tube.

• The luteal phase. Includes the remaining 14 days of the menstrual cycle (when a woman has a 28-day cycle), beginning with ovulation and ending with the start of the next menstrual period. Two other hormones are dominant during the luteal phase:
  

• Luteinizing hormone. Produced by the pituitary gland. Small amounts of LH are produced early in the follicular phase, but the later surge of LH causes ovulation. After ovulation, LH acts on the empty egg follicle, now called the corpus luteum. The corpus luteum secretes small amounts of estrogen and greater amounts of progesterone.
  
  
• Progesterone. Produced by the corpus luteum. Progesterone promotes the growth and development of the endometrium to receive a fertilized egg. The progesterone level peaks at about day 21 of a 28-day cycle. If there is no fertilized egg, progesterone levels decrease, causing the uterine lining to begin to break down after several days. This subsequently results in menstruation.  
  

The production of estrogen also decreases after ovulation, as the follicles degenerate. With less estrogen and progesterone to promote endometrial development, the uterine lining breaks down enough to be shed by the end of the luteal phase at day 28. This begins the next menstrual period and returns the menstrual cycle to day one.

The days in the middle of the menstrual cycle, near ovulation, are the most likely time for a woman to get pregnant during sexual intercourse. However, in some women ovulation occurs at different times in the cycle. Women can become pregnant while having their periods. The variability in menstrual cycle length in women makes the precise identification of where one woman is in her particular cycle difficult.

If the egg is fertilized in the fallopian tube, it typically continues to the uterus. The enriched endometrium provides a place for the fertilized egg to implant. It does not shed and leave the body and this missed period may be the first sign of pregnancy.

However, some women may experience a small amount of spotting or bleeding very early in the pregnancy, about 10 to 14 days after fertilization, which may be mistaken for menstruation. This is known as implantation bleeding and occurs when the fertilized egg first attaches to the lining of the uterus. This type of bleeding is usually a bit earlier and lighter in color than a usual period and does not last long.

Menstruation can cause problems ranging from mild discomfort to pain. Some body changes during the monthly cycle are normal, including breast tenderness or some discomfort just before menstruation.

Some women have symptoms before menstruating that are severe enough to be classified as premenstrual syndrome (PMS). Pain during menstruation (dysmenorrhea) includes menstrual cramps. They may be caused by hormone-like substances known as prostaglandins. Severe dysmenorrhea may also be a symptom of other conditions, including endometriosis or uterine fibroids. Most mild cramps can be treated with over-the-counter analgesic medication (painkillers) or with the use of heating pads or hot water bottles.

During menstruation, prolonged tampon use has been associated with an extremely rare infection called toxic shock syndrome (TSS). To avoid TSS, women should follow the tampon package instructions, change their tampons frequently and use the lowest absorbency tampon possible for their menstrual flow.

Amenorrhea is the lack of menstruation. Primary amenorrhea refers to girls who have not experienced their first periods. A girl who has not had her first period by age 16 should consult her physician or gynecologist. Secondary amenorrhea refers to someone who has menstruated previously but has stopped. The most common reason to miss a period is pregnancy, but other causes of missed periods may include:

• Breastfeeding
• Stopping the use of birth control pills
• Extreme weight loss
• Excessive exercise (e.g., ballet dancers, gymnasts)
• Stress
• Hormonal imbalances

Irregular periods may indicate hormonal imbalances or such conditions as polycystic ovarian syndrome (PCOS). Women may also experience very heavy bleeding (menorrhagia) or vaginal bleeding between periods (metrorrhagia). These may be symptoms of some sexually transmitted diseases such as gonorrhea, uterine abnormalities such as cancer, polyps or fibroid tumors. Menorrhagia and metrorrhagia may be controlled by birth control pills in women who are able to use this type of medication.

Women should consult their gynecologist about any menstrual conditions that change quickly or become progressively worse. Missed or irregular periods or abnormal vaginal bleeding also indicate a need for medical consultation.

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients or parents may wish to ask their doctor or healthcare professional the following questions regarding menstruation:

1. When can I expect to begin menstruation?
   
   
2. When will my periods become more regular?
   
   
3. What can I do to treat my discomfort during my periods?
   
   
4. How can I calculate the length of my menstrual cycle?
   
   
5. When will I be ovulating during my cycle?
   
   
6. Is it possible to become pregnant during my period?
   
   
7. How will I know if I’m experiencing PMS?
   
   
8. What signs and symptoms indicate a serious problem with menstruation?
   
   
9. Can dieting affect my period?
   
   
10. Am I a candidate to use birth control pills for regulating my cycle?
    
    
11. Will breastfeeding interfere with my periods?
    
    
12. When will my periods return to normal after the birth of my child?
    
    
13. What are the risks with tampon use?
    
    
14. How will my periods change when I approach menopause?