Go to the doc, if you really want to know.
Pulling out somewhat reduces the likelihood of pregnancy. But besides that, lemme tell you why I thought I couldn't get pregnant when I was a wee lass.
Many moons ago, my first boyfriend and I engaged in nightly intercourse for about a year. No pullouts, no condom. (Yeah yeah, I know.) And he wasn't sterile (went on to have kids a couple of years later). And I became pregnant about a year later from someone else. And I've been very fertile and easy to impregnate throughout my bearing years.
I don't profess to know the science behind it. I'm just saying don't be scared. Maybe it's chemistry, maybe it's just the odds playing.