Babies don't get teeth until they're over six months old, and early fetuses don't have enough strength to damage much of anything. The movement of a zombie fetus wouldn't look much different from the random squirming of a regular living fetus. Zombie fetus and regular fetus also share a lack of concern for the host mom or any understanding that they're even inside of another being. It's an interesting concept, but as soon as I gave it more than a few minutes' thought I realized that a fetus zombie is about the same as a living fetus and I couldn't figure out how a zombie fetus would be a threat.
Zombie baby & mom share blood. In the past we've seen scratches, cuts, etc turn into bad news. I guess it's possible, but not sure how they'd do it.