While a missed menstrual period is the hallmark of early pregnancy, missing a period can be due to a number of factors and conditions. The term amenorrhea is used by doctors to describe the absence of menstruation. Primary amenorrhea (in which a woman does not begin menstruating) is very rare, while secondary amenorrhea (absence of periods in a previously menstruating woman) is much more common. Periods may also be irregular, with skipped periods, for the first few years after a woman has begun menstruating and in the perimenopause (the time period preceding menopause). A woman is not considered to have secondary amenorrhea until she has missed three menstrual periods in a row. However, the causes of secondary amenorrhea are generally the same factors or conditions that can lead to a skipped period. If a woman is not pregnant, a missed menstrual period may be due to a wide variety of both physical and emotional conditions that range from lifestyle stress to rare and serious diseases. Many of the conditions that underlie missed menstrual periods interfere with the body's hormonal balance, particularly regarding levels of the sex hormones.