It IS possible to not gain much excess weight while you're pregnant, but it's hard. Think about how TOM hormones affects your appetite-and then multiply that by about 10. Atleast. On the good days.
That doesn't mean that you have to indulge, it just means it is going to be more tempting. I personally gained ALOT of weight with all 3 of mine.
You will lose 15-20 pounds within a few days after having a baby-the weight of the baby, placenta, amniotic fluid, etc.
While you're pregnant just isn't the time to be worried about creating a calorie deficit. If you're concerned and want to be vigilant about not undoing all your efforts losing the weight beforehand I'd definitely talk to a doctor or a nutritionist about how many calories you'd need daily while pregnant to maintain your weight (aside from the actual pregnancy gain of baby, fluids, etc). No (fat) gain, no loss is the ideal. If you only gain what you'll lose when you give birth then you've got it made.
Only you can know when you're ready to have a baby and there are so many other factors to consider besides just this, but no matter what after the fact it is very likely that you'll have some work to do after you give birth whether you lose all the weight first or not.
You
can choose and control your diet while you're pregnant and not have the pregnancy affect your weight after the baby is born. But hey-if it doesn't work out exactly that way it won't be the end of the world. You're already proving you can do it right now, you could do it again. And I promise that when you have your child, even if you have to work off an extra 20 pounds, you'll have no doubt that it was totally worth the extra effort.
There is also that little consolation prize called breastfeeding! That alone burns an extra 600 calories a day