I would suggest buying her some clothes from Gap Maternity online as they have a Free returns policy. It is important to remember that most women will take their normal size in Maternity clothing - as the clothing is designed for the pregnancy. If a woman has put on a lot of extra weight, then they may need to go up a size.
It is a very thoughtful thing for you to do for your wife, I am sure she will appreciate it.
best of luck to you both.
