Amenorrhea is the medical term to describe the absence of menstruation in women of a childbearing age (from the onset of puberty to menopause). It generally excludes any absent menstruation caused by normal body functions, such as occurs during pregnancy, breastfeeding and the period prior to menopause (perimenopause).

Amenorrhea is classified into two categories:

• Primary amenorrhea. When a girl has not had her first menstrual period (menarche) by age 16. It is also referred to as “delayed menarche.”
  
  
• Secondary amenorrhea. When a woman who has previously menstruated fails to menstruate for at least three months.

These terms are used only to describe the timing of amenorrhea and do not indicate cause.

Regular menstrual cycle begins when the brain's hypothalamus and pituitary gland release certain messenger hormones, such as gonadotropin (called GnRH for gonadotropin-releasing hormone), follicle stimulating (FSH) and luteinizing (LH) hormones. In response, the ovaries secrete estrogen and progesterone at different times to regulate the menstrual cycle.

Estrogen stimulates and promotes the growth of the endometrium. Progesterone, which forms after ovulation and is secreted from tissue known as the corpus luteum, transforms the rapidly growing endometrium into secretory endometrium. This will serve as the precursor for the future development of the placenta if pregnancy does occur. If pregnancy does not occur, this secretory endometrium breaks down and sheds during the ensuing menstrual period.

Amenorrhea can occur if problems occur with GnRH, the element that controls the hormones responsible for ovulation and the menstrual cycle (e.g., FSH, LH). A number of factors can cause the hypothalamus to decrease or stop releasing GnRH. This can result in inadequate production of estrogen or in progesterone and failure of ovulation.

An overproduction of male hormones (androgens) can also lead to amenorrhea. Too many androgens (as opposed to the normal balance between androgen and estrogen levels) results in a decrease in the pituitary hormones that lead to ovulation and menstruation.

In addition, the functions of the thyroid gland can affect a woman’s reproductive system, particularly if the thyroid is overactive or underactive. Thyroid disorders can affect the hypothalamus and disrupt the menstrual cycle causing a woman to have absent periods. Plus, thyroid disorders can have an effect on the production of the hormone prolactin, which can also result in a cessation in menstrual periods. 

In other cases, the hypothalamus, pituitary and ovaries all may be functioning normally, yet amenorrhea may occur as a result of adhesions or scars in the endometrial cavity. These scars prevent the normal buildup and shedding of the uterine lining, which can result in very light or absent menstruation.

Structural disorders can also play a role in amenorrhea. Chromosomal abnormalities may arise during fetal development that lead a female infant to be born without a major part of her reproductive system, such as the uterus, cervix or vagina. Or, a membrane or wall present in the vagina may block the outflow of blood from the uterus and cervix. In these cases, women will undergo puberty but will not experience menstruation.

Regular menstrual cycles are a sign of good health. They mean that a woman’s reproductive system is working properly. Women who miss their periods (unrelated to pregnancy) may want to talk with a physician to make sure they do not have an underlying condition.

In addition, women who get fewer than six or eight periods a year (oligomenorrhea) may also want to consult with a physician. Oligomenorrhea may be common for girls in early puberty, but after this period it may indicate a hormonal imbalance such as polycystic ovarian syndrome.

Amenorrhea and oligomenorrhea have been associated with long-term health consequences. For instance, the loss of menstrual regularity has been associated with increased risk of uterine cancer (because the lining of the uterus is not shed monthly), reduced bone density and increased fracture rates, as well as future fertility concerns.