Amenorrhoea is the absence of periods (menstruation). Usually, but not always, it also means no eggs are produced (anovulation). In many cases, as in girls before puberty, or during pregnancy, breastfeeding or the menopause, this is normal and is called physiological amenorrhoea.

An associated condition known as oligomenorrhoea refers to infrequent periods, usually every three to six months which may be irregular and long.

Primary amenorrhoea is the failure to establish menstruation. It is regarded as abnormal if a girl has not started sexual physical development and menstruation by the age of 14, or if a girl with normal sexual physical characteristics has not started menstruation by the age of 16. Primary amenorrhoea affects only about 3 girls per 1000.

Secondary amenorrhoea is defined as the absence of menstruation for six consecutive months in a woman who previously had regular periods. Secondary amenorrhoea implies a gynaecological disorder or a general cause or disease.

Secondary amenorrhoea affects about 3% of girls. Up to 50% of competitive long-distance runners (running 80 miles per week) and up to 44% of ballet dancers have amenorrhoea.