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The great present interest of the public and of at least a part of the medical profession in the subject of birth control has caused recent publications in the popular and medical press concerning the rhythmic or "natural" method of contraception to attract considerable attention.
This little monograph sets forth the hypothesis on which the author bases his technic for contraception and his method of indicating that period of the menstrual cycle during which conception can occur, and, conversely, that portion of the cycle during which impregnation may confidently be expected not to occur.
It has gradually come to be accepted that the life period and certainly the duration of the fertility of both the ovum and the sperm are less than was formerly supposed to be the case. Knaus discusses at some length the factors which govern the length of life of human sperms and concludes, on apparently sufficient
Periodic Fertility and Sterility in Woman: A Natural Method of Birth Control.. Am J Dis Child. 1935;49(5):1391-1392. doi:10.1001/archpedi.1935.01970050289028