The answer is more complicated than simply missing a dose, or failing to take your birth control at just the right time each day. Scientists are just beginning to understand how individual differences in body chemistry can affect how well the Pill works.
After getting pregnant twice while taking birth control, Amanda Mascarelli wrote about the different factors that can affect Pill performance.
"We know that some people are fast metabolizers and some people are slow metabolizers," says Alison Edelman, an associate professor of obstetrics and gynecology at the Oregon Health and Science University in Portland. A fast metabolizer might pass a drug through her liver too quickly; in the case of the pill, this could lower the hormone level below the critical threshold.