The natural fertility of any population is affected by the extent of primary and secondary sterility among women or couples, for a certain percentage of women or couples is unable to participate in reproduction either throughout their lives or after haying one or more children.

The average number of children ever born per woman depends upon the percentage of women or couples who suffer from primary sterility that is couples who have never had any children.

In India, the incidence of childlessness among couples, who have passed the reproductive Principles of Population Studies age child, is found to be less than in some developed countries.

The extent of primarily sterile women in India was 6 per cent among those whose age at marriage was below 15 years, and 10.5 per cent for those whose age at marriage was above 25.

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It, therefore, appears that the extent of primary sterility also varies with the female age at marriage. In 1961, it was found in the Post-Census Fertility Survey in Gujarat, Assam, Tamil Nadu and Uttar Pradesh that the percentage of childless married women at the end of their reproductive period was between 4.2 and 6.6 in rural areas and between 4.4 and 6.7 in urban areas.

The magnitude of fecundity-impairment or of secondary of sterility that is, the inability of women who do not practice contraception to participate in reproduction after getting one or more children is also an important physiological factor affecting the fertility of any population.

The onset of secondary sterility may be natural or may be due to certain pathological conditions or some affection or accident during childbirth.

The study of secondary sterility is important in estimating the future growth of families. Though some studies on the extent of secondary sterility have been made in India, their exact contribution to the relatively low Indian fertility is not known.