Oh sh*t is right. As everyone who's ever taken a high school sex ed class knows, it's impossible to get pregnant from having anal sex. Sure, you may die instantly from an STD (if your ex-marine gym teacher is to be believed), but at least you won't have to worry about having a baby. Right?
Dr. Steixner, now the Director of the Institute of Men's Health at Jersey Urology Group in Atlantic City, was a mere medical student when he studied a female patient who had a "cloaca." While a common part of reptile and bird anatomy, in humans a cloaca is a very rare condition in which a woman's underdeveloped bladder, vagina and rectum open through one chamber.
When this woman came back to the ER clearly pregnant, Dr. Steixner says,
We knew about her condition, and we had followed her for a decade. After doing a whole bunch of X-rays, we determined that she got pregnant from having anal sex. Was she going to have to poop the baby out? I thought that would be hilarious, but ultimately, the OBGYNs couldn't figure out a way to effectively deliver the baby through the rectum safely. So she had a C-section.
So, yes, there's a very, very slim chance a woman could become pregnant through the backdoor. In that case, though, you likely have bigger problems on your hands -- like telling your vagina from your rectum. Let's just be thankful the worst thing we'll be delivering through our butts is Chipotle.