Recent evidence indicating that the actual union of sperm and egg in the human species is possible only for a period of less than twenty-four hours in any menstrual cycle has stimulated tremendous interest in the possible practical applications of this apparently biologic fact. The principal corollary, according to Willson1 in a review of this subject, is that intercourse, in order to be fertile, must take place not more than twenty-four hours preceding the actual occurrence of ovulation. This view is revolutionary and runs counter to the conclusions drawn from the more or less continuous sexual activity of man and the primates as opposed to lower animals. Furthermore, it conflicts with the previous understanding of the biologic analogies of human menstruation. Clearly, according to Willson, the biologic classification of human beings lends credence to the available clinical and laboratory evidence which points to the same midcycle timing of ovulation
THE RHYTHM OF HUMAN FERTILITY. JAMA. 1937;108(23):1972–1973. doi:10.1001/jama.1937.02780230032011
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