My wife did one this afternoon with an experienced PFA. He said the worst case is to undo it by 8/15/18 when we file tax return with extension, if 孙 is still unborn then, i.e., no real SSN.
Most likely though, it only matters when the money in the account is taken out many years later, we then need a real 孙 with a real SSN and going to a real college.
My wife think with 11 months left it is very hopeful we will get a 孙 from 2 D-inlaws or our D.
Now let's see whether her incentive works...