The fertile days in the menstrual cycle were discovered in the 1930s, by gynaecologists Hermann Knaus (Austria) and Kyusaku Ogino (Japan). This laid the foundation for modern methods of self-observation. Numerous aids were developed to calculate the days on which a woman could become pregnant, which were few in number.
Knaus’ and Ogino’s findings, while useful for purposes of both conception and contraception, are not able to ensure complete effectiveness: the menstrual cycle, being influenced by the emotions and other factors, is not always regular.
With regard to contraception, one must remember that sperm cells are extremely hardy and can survive in the Fallopian tubes for up to five days, during which time they are capable of fertilising an egg.
For this reason the rhythm method of birth control is suitable solely for couples who want to have a child eventually and are merely delaying conception.