Please dont take this the wrong way but, If you were lucky enough to be with this girl why the hang up about the skin?? As a woman of 31 who has had a child and lost weight quite quickly in the past I can relate to her problem ( or should I say, yor problem)
When I went from 11st to 8 1/2 stone I lost a lot of weight from my breasts, as this is very fatty tissue usually it is the first place that women lose the pounds.
Regarding her stomach, you have to understand that the type of skin you have dictates on how your body reacts to weight loss or carrying a child. I do not have a great deal of elastin in my skin, therefor when I had my son I got stretch marks.
And when I lost my weight I do have loose skin on my lower abdomen. There is nothing you can do about it besides surgey. Exercise will tighten your muscles making it appear a tad better but once the skin has been damaged, there isnt anything you can do. I do believe that because you have already asked her if she has had a baby that she will now be feeling very uncomfortable about the thought of getting naked again in front of you. Be honest with her, If she doesnt like the question its no loss but if you dont ask then everytime you are with her she will see that you are uneasy.... Honesty is always best