Would you want to have a very expensive dress custom fitted for your body knowing that you will gain weight and stretch it out?
Your body is the most precious garment you own. A significant portion of the abdominoplasty involves repairing the muscle to enhance and create the impression of a waistline. Pregnancy will undoubtedly undo the muscle repair. Why waste the time, money effort, healing and surgery if you know you will intentionally damage it? You can do this if you want to enjoy the temporary nature of the results but realize that it will only be short term. It is very likely you will have to repeat the tummy tuck after the pregnancy.
I have performed this procedure on young adults who have experienced masssive wieght loss in their late teens or early 20's but only after having a long discussion about the inevitability of having to repeat the procedure and only if they have long term intentions of getting pregnant