I loosely followed SW whilst pregnant and put on exactly 2 stone, of which I lost all but 2lbs the day after delivery, then went up by 5lbs until last week (9 weeks after delivery) when I rejoined class. So if you look at it as a 5lb gain it's not too bad.
If you eat sensibly you wont gain too much. Pregnancy weight gain isn't all fat either, you gain placental weight, breast tissue, extra blood etc so you do have to expect a upward trend in the scales regardless but it's NOT fat. Hope this helps.