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The age of menarche.


 In females, the first menstrual period, menarche, signals the beginning of the capacity to reproduce and is associated with the development of secondary sexual characteristics. Menarche is one of the most significant milestones in a woman's life. The first cycles tend to be anovulatory and vary widely in length. They are usually painless and occur without warning. Menarche occurs between the ages of 10 and 16 years in most girls in developed countries. Although the precise determinants of menarcheal age remain to be understood, genetic influences, socioeconomic conditions, general health and well-being, nutritional status, certain types of exercise, seasonality, and family size possibly play a role. Over the past century the age at menarche has fallen in industrialized countries, but that trend has stopped and may even be reversing. The average age at menarche in 1840 was 16.5 years, now it is 13. The age at menopause, however, has remained relatively constant at approximately 50 years. The length of time during which women are exposed to endogenous estrogen has therefore been increasing. Reasons for the fall in menarcheal age remain unclear, but one interpretation considers it to be a reflection of the improvement in health and environmental conditions. The decline in menarcheal age appears to be leveling off in many countries such as Britain, Iceland, Italy, Poland, and Sweden, but continues in Germany and some other countries. Late menarche is associated with a decreased risk of developing breast cancer in later life, a decreased frequency of coronary heart disease, later first pregnancy, and reduction in teen pregnancy. Late menarche may, however, be positively associated with the risk of developing Alzheimer's disease.
