Now hailed as one of the most important inventions of the 20th century, the birth control pill almost never came to be. Back in the 1950s, contraception was illegal in much of the country, and “making a pill that would help make women more equal was something nobody had an interest in doing,” says Jonathan Eig, a Lake View writer and the author of Get Capone and Luckiest Man. His latest book, The Birth of the Pill (W. W. Norton, $28), tells the tale of four rebels who “were able to pull off something that was seemingly impossible”—and shares many other surprising facts.

1. The pill was originally envisioned for poor women. Margaret Sanger, the fiery founder of Planned Parenthood, had witnessed miserable conditions among immigrant women while working as a nurse in New York City and was looking for a way to help them control their lives and lift themselves out of poverty. But the pill found a profitable market among wealthier, more educated women and has been most popular in developed countries.

2. It was invented by just four renegades. Sanger, of course; Gregory Pincus, a young biologist who had been fired by Harvard University after he achieved in vitro fertilization in rabbits; Katharine McCormick, a wealthy ally of the women’s movement who funded the bulk of the research; and John Rock, a debonair Catholic doctor who went up against the church to win public support.

3. It was tested mostly on Catholic women. Identifying willing test subjects close to home proved difficult. But in Río Piedras—an overpopulated, poverty-stricken district of San Juan, Puerto Rico—Pincus and Rock found hundreds eager to try the drug.

4. A Chicago-area pharmaceutical company commercialized it. While most firms shied away from controversial birth control research, Skokie’s G. D. Searle & Company (later acquired by Pfizer) agreed to partner with Pincus. In 1960, the pill (originally sold under the name Enovid) received approval from the Food and Drug Administration. Incidentally, the now-iconic circular dispenser was also created in the Chicago area, in west suburban Geneva, by David P. Wagner. Wagner, an engineer, wanted to help his wife remember to take the pill after the birth of their fourth child.

5. Its primary inventor never profited much from its discovery. Eig writes that the bushy-browed Pincus “made the decision early in his career that science mattered more than money.” He did not patent the pill, thereby forgoing millions in potential earnings.