Yes I do have a source about moving pregnat sims. It's happened to me more than once. My children would not age I had to delete them out of the game as it was preventing other children in my game from not aging up. Once I got rid of the children that cause the corruption all the rest of the children would age up. Why I have this problems and you don't ??? I even actually had this problem on an old game back from 2010 too seperate from this newer game I have.
So my advice is if she does not know if it will corrup her children or not better safe than sorry. Whats the big deal about letting that child age up a little and then merge. You can always use the cheat by taking them into cas if you dont like the names or traits the computer gves you.