No.
The chances are so extremely low that you may as well consider it a non-item :) his semen would have to trickle down her labia them
up into her vagina then all the way up through the cervix and then make it's regular journey towards the fallopian tubes.
Unless you don't go to the bathroom afterwards (and girls, you really should be. It helps prevent uti's) there is basically no way in the world you're going to get pregnant.
I'm sure ther have been freak accidents, but I bet she didn't clean herself afterwards!