Definition of Secondary amenorrhea:

Secondary amenorrhea is lack of periods for 6 or more months in a woman who has already started menstruation and who is not pregnant, lactating, or in menopause.

See also:

Menstruation - Absent

Primary amenorrhea

Causes, incidence, and risk factors:

About 4% of women have secondary amenorrhea. There are many reasons for missing a period.

You are at an increased risk of secondary amenorrhea if you engage in extreme and prolonged exercise (particularly without adequate conditioning), have less than 15 - 17% body fat, are obese, or take hormonal supplements.

Anxiety and emotional distress can also cause a missed period.

The following drugs may also cause missed periods: 

• Busulfan
• Chlorambucil
• Cyclophosphamide
• Phenothiazines 
• Oral contraceptives
• Non-oral contraceptives such as Depo-Provera

Also, procedures such as a dilation and curettage (D and C) can cause a woman to develop amenorrhea if scar tissue develops within the uterine cavity.