Amenorrhea means that a woman of childbearing age fails to menstruate. The normal menstrual cycle ranges from 23 to 35 days. It is regulated by the hypothalamus, a portion of the brain that also controls body temperature, appetite and blood pressure. The hypothalamus stimulates the pituitary gland (located near the base of the brain) to release two hormones that regulate female reproductive cycling: luteinizing hormone and follicle-stimulating hormone.

These hormones, in turn, influence production of the hormones estrogen and progesterone, which are responsible for cyclic changes in the lining of the uterus, including menstruation. In order for a woman to have regular menstrual cycles, her hypothalamus, pituitary gland, ovaries and uterus must be functioning properly. Her cervix and vagina must also be anatomically normal to allow the passage of menstrual flow.

There are two types of amenorrhea:

Primary amenorrhea is defined as no period by age of 14 in the absence of growth/secondary sexual characteristics or no period by age of 16 regardless of presence of normal growth and development with appearance of secondary sexual characteristics.Primary amenorrhea is most often due to late puberty, which is fairly common in teenage girls who are very thin or very athletic. These young women are typically underweight, and their bodies have not experienced the normal puberty-related rise in body fat that is required to trigger the beginning of menstrual cycling. In other girls, the delay of menses may be due to Turner's syndrome, a genetic disorder involving the sex chromosomes, or to a developmental abnormality of the female reproductive organs.

Secondary amenorrhea is absence of periods for a length of time equivalent to a total of at least 3 of the previous cycle intervals or 6 months of amenorrhea. Secondary amenorrhea can be caused by:

• Pregnancy (the most common cause)
• Breastfeeding (lactation)
• Menopause (the normal age-related end of menstruation)
• Premature ovarian failure (menopause before age 40)
• Hysterectomy (surgical removal of the uterus)
• Use of a long-acting progesterone, such as Depo-Provera, for birth control
• Tumors of the pituitary gland, especially prolactinomas
• Polycystic ovary disease (a condition that causes abnormal levels of estrogen, luteinizing hormone and other hormones)
• Endocrine disorders such as Cushing's syndrome (in which there are very high levels of cortisol, an adrenal hormone) or hyperthyroidism (abnormally high levels of thyroid hormone)
• Emotional or physical stress
• Rapid weight loss
• Obesity
• Frequent strenuous exercise
• Chronic illness, such as colitis, kidney failure or cystic fibrosis
• Chemotherapy for cancer
• Cysts or tumors in the ovaries