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The fecundity of a couple measures their capacity to have children. Fecundity varies widely from one individual and from one couple to another. Beyond such individual variations, fecundity in women reaches its maximum at about 20, and then decreases slowly to 35, and then more rapidly after 35 until it becomes zero at about 45 or 50.
The fecundity of couples was estimated by observing recently married couples in societies in which sex before marriage was forbidden and who did not practice birth control or limitation of births. The rate of arrival of the first children then depended solely on their fecundity. In current societies in which limitation of births is in wide use, it is difficult to measure the fecundity of couples. On the one hand, the improvements in the state of health of populations very probably leads to an increase in fecundity; on the other hand, it is possible that sexually transmissible diseases become more frequent and that pollution leads to a drop in fecundity.