The term amenorrhea is most often used by physicians to describe the absence of menstruation. The absence of menstruation is most often caused by normal body functions such as:

• Pregnancy. This is the most common cause of amenorrhea in women of reproductive age. When a woman’s egg is implanted in the lining of the uterus, the lining remains to nourish the fetus and is not shed by menstruation.
  
  
• Breastfeeding. Women who lactate often experience amenorrhea for up to six months.
  
  
• During perimenopause and/or after menopause. Some women may experience early menopause (before age 40). The lack of normal ovulatory function associated with menopause – defined as the cessation of menses for 12 months which then becomes permanent – decreases the amount of estrogen in a woman’s body, which results in menstruation stopping. Some women may experience irregular periods before complete amenorrhea.

Some women who take birth control pills may not have periods when they first begin taking the medication. It can take up to three months before a cycle becomes evident. With some low-dose birth control pills, women may experience a day or two of light spotting instead of a regular period. This is normal because the low dose of hormone in the pill results in the development of only a very small amount of lining in the uterus. Menstruation usually resumes after about three months. Also, when oral contraceptives are stopped, it may take up to six months for a regular menstrual cycle to occur. Contraceptives administered by injections, patches or those that are implanted may also cause the temporary cessation of menstrual periods.

In addition, progesterone-containing intrauterine devices (IUDs) may cause amenorrhea in some women. However, women who experience prolonged amenorrhea while on birth control should contact their physician to rule out pregnancy. Other causes of amenorrhea that are not associated with normal gynecological function include the following:

• Excessive exercise. Women who regularly participate in sports that require rigorous training (e.g., ballet, gymnastics) may fail to menstruate. Other factors combined with the excessive physical activity may also contribute to the loss of periods in athletes, including low body fat, stress and high-energy expenditure. Excessive exercise requires vigorous training several hours per day and when this is combined with inadequate intake of calories, vitamins and minerals, amenorrhea may result.
  
  
• Low body weight or obesity. Excessively low or high body weight interferes with many of the body’s hormonal functions and may interrupt ovulation. For example, malnourished women or those with eating disorders (e.g., anorexia nervosa, bulimia) or excessive body fat (obesity) often stop having periods due to abnormal hormonal changes.
  
  
• Stress. Mental stress can temporarily disrupt the functioning of the hypothalamus gland, which may result in the cessation of ovulation and menstruation. However, regular menstrual periods usually resume after a reduction of stress.
  
  
• Polycystic ovarian syndrome (PCOS). Altered hormone levels, such as occurs when PCOS is present, may interfere or prevent ovulation, resulting in amenorrhea. With PCOS, women may have high levels of male hormones (androgens) present that interfere with ovulation and the menstrual cycle. Women with PCOS may menstruate infrequently (oligomenorrhea) or not at all.
  
  
• Chronic illness. Diseases that affect the immune system such as diabetes, HIV or cancer may interfere with a woman’s menstrual cycle. Epilepsy (a chronic disorder of brain function that causes seizures) may also result in amenorrhea.
  
  
• Medications. Some medications may cause a woman’s menstrual period to cease. For example, antidepressants, antipsychotics, some chemotherapy drugs and oral corticosteroids can cause amenorrhea.
  
  
• Cigarette, drug and/or alcohol abuse. The constant use of recreational drugs, such as cocaine or marijuana, has central effects that may disrupt a woman’s menstrual cycle. These drugs may result in low follicle stimulating hormone (FSH) and luteinizing hormone (LH) levels and slowed GnRH release which interferes with ovulation. Excessive cigarette smoking (due to its anti-estrogenic effects) has also been associated with lack of menstruation. In addition, malnutrition and cirrhosis (a potentially life-threatening condition that affects liver function) associated with alcoholism may cause loss of menstrual regularity. 
  
  
• Turner syndrome (TS). A rare chromosomal abnormality that affects females and causes a premature depletion of the eggs and follicles involved in ovulation and menstruation. Women with TS do not have proper ovarian development and may experience delayed menarche or no menarche at all.
  
  
• Lack of reproductive organs. In some cases, problems arise during fetal development which may lead a female infant to be born without some vital part of her reproductive system, such as the uterus, cervix or vagina. In such cases, women with reproductive systems that did not develop normally will not have menstrual cycles.
  
  
• Structural abnormalities of the vagina. Sometimes there may be irregularities in the structure of a woman’s vagina that are present at birth but may go unnoticed until puberty. For example, a membrane or wall that blocks the outflow of blood from the uterus or cervix may be present.
  
  
• Tumors in the hypothalamus or pituitary gland may also interfere with secretion of hormones and result in amenorrhea.
  
  
• Thyroid disorders. An underactive thyroid gland (hypothyroidism) or an overactive one (hyperthyroidism) commonly causes menstrual irregularities, such as amenorrhea. In addition, thyroid disorders may cause an increase or decrease in the production of the hormone prolactin, which can affect the hypothalamus and disrupt a woman’s menstrual cycle.
  
  
• Asherman syndrome. The buildup of scar tissue in the lining of the uterus, which can sometimes occur after certain uterine procedures, such as dilation and curettage (D&C), Caesarean section (C-section) or treatment for uterine fibroids, may prevent the normal buildup and shedding of the lining of the uterus. This condition can result in very light or absent periods.

It is important to note that several hormonal disorders that result in the inability to regularly shed the endometrial lining of the uterus could put a woman at risk for developing uterine cancer. In addition, if left untreated for over 12 months it may also cause loss of bone density which can lead to osteoporosis.