The absence of menstrual periods is called amenorrhea. Primary amenorrhea is the failure to start having a period by the age of 16. Secondary amenorrhea is more common and refers to either the temporary or permanent ending of periods in a woman who has menstruated normally in the past.
Many women miss a period occasionally.
Amenorrhea occurs if a woman misses three or more periods in a row. In the first instance no menses have started by the age of 14 years in the absence of secondary sexual characteristics. In the second instance no menses have started by the age of 16 years but there is normal development of secondary sexual characteristics. Secondary amenorrhoea means the cessation of menstruation for at least 3 consecutive months in a female who has previously menstruated. The condition occurs only in females from puberty to menopause and is characterised by the absence of menstrual periods for 3 or more months in a female who has menstruated at least once.