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Document Type

Article

Abstract

The frequency distributions for age at menarche and for age at menopause are given for 324 women whose menstrual histories have been recorded from their university student days ( 1934-38) until natural menopause terminated their potential for reproduction. Age at menarche is not correlated with age at menopause. The difference between these two ages is accepted as measuring the maximum possible length of potential reproductive life. A mean age at menarche of 13.6 years, and at menopause of 49.5 years, gives an average length of menstrual life of 35.9 years for the cases studied. Variation in age at menopause is shown to be 2.2 times that of age at menarche. Since these 2 ages are independent, length of menstrual life is even more variable than age at menopause, with which it is strongly correlated (r =-f-0.91). Age at menarche influences length of menstrual life only slightly (r = -0.38). Age at menarche is well known to show secular change. Data accumulating within this research program suggests that average age at menarche in the USA may have begun an upswing in recent decades. Maximum reproduction potential, attainable only in theory by assuming all menstrual cycles produce ova of undiminished capacity to become fertilised, can be defined by combining the cumulative frequency curve for age at menarche with the inverse of that for age at menopause. The influence of “anovulatory menstrual cycles” in reducing that potential is illustrated using Vollman’s conclusions from his extensive basal body temperature histories. All pregnancies as well as breast feeding operate to reduce fecundability. The various contraceptive measures used by mankind also lower reproductive performance until the fecundability curve becomes the curve of realized fertility. The collective impact of pregnancy and contraceptive practises in lowering the reproduction expectancy curve is vised as a measure of control of conception.

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