Amenorrhea is the transient or permanent absence of menstrual flow. It may be subdivided into primary and secondary presentations, relative to menarche: 
Primary amenorrhea: lack of menses by age 15 years in a patient with appropriate development of secondary sexual characteristics, or absent menses by age 13 years and no other pubertal maturation
Secondary amenorrhea: lack of menses in a nonpregnant female for at least 3 cycles of her previous interval, or lack of menses for 6 months in a patient who was previously menstruating.
Although overlapping attributes exist between the two groups, the diagnostic approaches vary significantly. The prevalence of primary amenorrhea in the US is <0.1%, compared to 4% for secondary amenorrhea.  
Despite the low prevalence of secondary amenorrhea, a prompt, comprehensive assessment is warranted unless the patient is pregnant, lactating, or using hormonal contraceptives, as amenorrhea is often the presenting sign of an underlying reproductive disorder. A delay in diagnosis and treatment may adversely impact the future of such patients. For example, in polycystic ovary syndrome (PCOS) and hyperinsulinemia, behavioral and dietary modifications may prevent subsequent cardiovascular disease. 
Division of Urogynecology
Center for Minimally Invasive Gynecologic Surgery
Cedars-Sinai Medical Center
MJS reports that he is a consultant for Medtronic, and he is on the advisory boards at AbbVie and Allergan.
Department of Obstetrics, Gynecology, and Reproductive Sciences
Division of Reproductive Endocrinology and Infertility
University of Pittsburgh
JSS declares that he has no competing interests.
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