Career
His 1926 book Het volkomen huwelijk (The Perfect Marriage) made him an instant international celebrity. The book advocated knowledge and sensuality in erotic life. The English translation, Ideal Marriage: Its Physiology and Technique was the best-known work on its subject for several decades, and was reprinted 46 times in the original edition, selling well over a half-million copies.
He is credited with showing in 1905 that women only ovulate once per menstrual cycle.
This contributed to the improvement of calendar-based methods of birth control, and later to the creation of other fertility awareness systems