Amenorrhoea is the transient or permanent absence of menstrual flow and may be subdivided into primary and secondary presentations, relative to menarche. 
Primary amenorrhoea: 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 amenorrhoea: lack of menses in a non-pregnant 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. Primary amenorrhoea is rare: about 0.3% of women fail to establish menstruation.  The prevalence of primary amenorrhoea in the US is <0.1%, compared with 4% for secondary amenorrhoea.   Even when causes of primary and secondary amenorrhoea overlap, the relative likelihoods of these aetiologies may differ. For example, polycystic ovary syndrome may cause either primary or secondary amenorrhoea, but much more commonly causes secondary amenorrhoea. Many causes of primary amenorrhoea are rare in the general population (e.g., Kallman syndrome). Conditions that may seem to be rare events generally may appear more commonly in this subgroup of adolescent girls presenting with primary amenorrhoea.
Despite the low prevalence of primary amenorrhoea, a prompt, comprehensive assessment by a consultant in reproductive medicine is warranted, as amenorrhoea is often the presenting sign of an underlying reproductive disorder. A delay in diagnosis and treatment may adversely impact the long-term future of such patients. For example, an adolescent with androgen insensitivity syndrome requires counselling for eventual removal of gonads, because these patients carry up to about a 30% risk of gonadal neoplasms. In polycystic ovary syndrome and hyperinsulinaemia, behavioural and dietary modifications may prevent subsequent cardiovascular disease.   
Division of Urogynecology
Center for Minimally Invasive Gynecologic Surgery
Cedars-Sinai Medical Center
MJS is a consultant for Medtronic, serves on the advisory boards of Abbvie and Allergan, and has been a moderator for Bayer.
MJS would like to gratefully acknowledge Jessica Papillon-Smith, a previous contributor to this topic. JP-S declares that she has no competing interests.
Department of Obstetrics, Gynecology, and Reproductive Sciences
Division of Reproductive Endocrinology and Infertility
University of Pittsburgh
JSS declares that he has no competing interests.
Head of Section
Reproductive and Maternal Medicine
University of Glasgow Royal Infirmary
MAL declares that she has no competing interests.
Professor and Chair
Department of Neurology
Wake Forest University School of Medicine
RS declares that he has no competing interests.
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