Hi S.,
Because of a previously existing medical condition I had to take oxycontin and percocet during my pregnancy. I am now the mother of a beautiful three month old boy who weighs 14 pounds, loves to smile (especially at the ladies), is fantastically alert and responsive and has no health issues except the sensitive skin that goes along with having red hair and blue eyes! (please excuse my first time mom gushing- I can't seem to help myself :))
From my experience I can tell you that being in pain can in fact be more stressful for your baby in utero than taking a moderate dose of a narcotic pain reliever. I actually had to take pain relief because my pain instigated preterm labor and the medication actually protected both me and my baby from the physical stress that was so hard on us both - a surprise to me as I had believed my stoicism was protecting my pregnancy. I am not telling you this to scare you-- far from it. I am an unusual case, but I am sharing my story with you to let you know that the prevailing wisdom that going 'natural' is always best for your pregnancy is not always right, and to reassure you that my baby--who was exposed to much more than the amount of pain reliever you would be taking--is now, as his pediatrician likes to say, 'the picture of infant health.' Most medications are Class C because, and rightly so, it is unethical to do clinical tests on pregnant women. In the end, you just have to take the information you have at hand and listen to your intuition.
Whatever you choose, the best advice I can give is to make your decision and then let yourself be without agonizing. Extra anxiety isn't good for any of us, even when we aren't sharing our bodies with another little person! Good luck and I hope your ankle feels better soon!