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Coital frequency among white, continuously married couples increased by 19 percent between 1965 and 1970, but by only five percent between 1970 and 1975. All of the increase in the latter five-year period is connected with a greater concentration of women in pregnancy-exposure and contraceptive-use categories associated with higher coital frequency. Over the five years, individual women who switched to more effective methods did not, on the average, experience as much of a decline in coital frequency as those who changed to less effective methods and those who did not change methods. (Some decline would be expected to be associated with the five-year increase in age.)
Trussell et al. (Mon,) studied this question.