The time it would take for the placenta to "reattach" the fetus would suffocate. Remember, you can only go without oxygen for something like 3 minutes. The surgery alone would take at least 30 (I'm thinking of how long it takes to sew in an aortal graft - JUST the sewing in part, not including the flushing, testing, or anything else).

I would imagine a tiny fetus could not even go THAT long without O2.