You’re about six beers deep and finally—after several overpriced dinners and cocktails—you’ve convinced a super hot chick that you’re the all-around awesome guy she initially hoped you'd be. Score! So as you head back to your place and things get heated, there’s no shot you’re killing the mood with a condom...right?

Bad idea. Terrible idea. In fact, that logic is so flawed, we’re going to go ahead and counter it with the ultimate mood killer: In the first year of a baby’s life—between doctor visits, clothing, food, and everything else, a parent can expect to spend more than $10,000. (Yeah. Let that sink in.)

So unless you’re ready to trade beer for baby formula? Read this—our expansive guide to making sure you don’t knock her up. Because even if you play it safe most of the time, you’re probably screwing up in ways you didn’t even know were possible. And like your old man always said, it only takes one mistake.

1. The Mistake: You think having sex standing up prevents pregnancy.

The Potential Damage: We couldn’t believe that nearly 1 in 5 of men buy into this, but according to a 2010 study, it's true. Yikes. “The law of gravity is not a contraceptive,” says Debby Herbenick, Ph.D., the co-director of The Center for Sexual Health Promotion. That’s a seemingly obvious assessment, right? Use it.

2. The Mistake: You blindly grab some lube.

The Potential Damage: Always, always, always check what your lube is made out of—otherwise, that condom might do jack. “Oil-based or petroleum-based lubricants like mineral oil or massage lotion can destroy condoms, making them more likely to break,” says sexologist and sexuality educator Megan Andelloux.

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