This trophy is extremely simple, and can easily be done while going though the story.
*** Spoiler - click to reveal ***When getting the girls' help, they will ask you to go into Planned Parenthood. When leaving the Planned Parenthood building Nazi Zombie Fetus enemies will attack. After battling your first group of fetuses, simply fart (press up on the Right Analog stick) on one of the fetuses, and the trophy will pop.