Over the centuries, lovers have tried all kinds of unappealing tricks to prevent pregnancy. There was crocodile dung in ancient Egypt, Aristotle’s recommendation of cedar oil, and Casanova’s method of using half a lemon as a cervical cap.1 But even the obvious modern alternative to the pill, condoms, have a failure rate. Because people don’t tend to use condoms exactly as they’re supposed to—they sometimes rip or slip—with the result that for every one hundred sexually active women using condoms for a year, eighteen will become pregnant.
