Or, you had sex while using a condom, hormonal birth control or an IUD, like a certain number of people do. https://www.webmd.com/sex/birth-control/birth-control-failure-why
According to this WebMD article, "But about 5% of the time, women using reliable birth control find themselves unexpectedly pregnant." That means for every 10,000 couples using birth control in a given year, there should be 500 pregnancies. For your reference, there are more than 60,000,000 women of childbearing age in the US. Assuming that even a third of them are sexually active but using birth control, we should see about a million pregnancies DESPITE the use of birth control (20 million x .05).