The menstrual cycle, while normally a smooth, regular process, is a complex phenomenon whose usual operation can break down at any point along the way. Menstrual disorders can be roughly divided into four categories: amenorrhea, dysfunctional uterine bleeding, dysmenorrhea, and oligomenorrhea.

The complete absence of menstrual periods is amenorrhea. There are two categories within this disorder: (1) primary amenorrhea, the relatively rare failure to start having a period by age 16 (the average age is 12), and (2) secondary amenorrhea, which is more common and refers to the temporary or permanent ending of periods in a woman who has menstruated normally in the past. Many women miss a period occasionally, but missing three or more periods in a row is considered amenorrhea. Prolonged amenorrhea can lead to infertility and other medical problems such as osteoporosis (thinning of the bones).

The absence of menstrual periods is a symptom, not a disease...