Stop obsessing. You and your doc. have already decided this will happen, and they've done it many times before. So all you have to do is convince yourself: IF the great big turds come out, a wee finger, gloved in smooth latex and lubed with the same KY she uses for Greek can most certainly go in with no trouble.
In fact if you give it a moment's calm reflection, you could imagine feeling the same comfortable release you get from dropping one of those doozies. Speaking of which, for your own peace of mind take care of that stuff that morning, and follow with a thorough shower and a real clean between your cheeks. When you're on your side with your pants down, the last thing you want to be distracted by is whether your bum smells and you're holding back an avalanche.
What you want to think is, "If so many folks people think this kinda byplay is fun, how bad can it be?"