To please the Catholic Church. The original birth control packs featured three weeks of active pills followed by one week of sugar placebos, so that a woman taking them experienced a 28-day cycle, which is about the average length of a menstrual cycle. But this is not medically necessary; it could have been any number of days. The birth control pill’s co-inventor, John Rock, was a devout Catholic. He thought that if the artificial cycle mimicked a natural one, it’d earn the Church’s approval. He was wrong: A papal commission set up by Pope John XXIII in 1963 voted in favor of Rock’s wishes to allow contraception for married couples, but Pope Paul VI shot it down two years later. It remains prohibited.