You approach a girl because you find her hot! When you ask them to marry you I hope you have found a lot more things to like about them than just the fact they are hot!
How dare she gain a couple pounds while carrying "your" child and preparing to drop 7-10 lbs through an opening she used to lie to you about your prick actually being a tight fit! Buckle up buddy, cause how it is your turn to return the favor and weather you believe it or not, you have to let her know she is the sexy woman you first met. Now is not the time to nag her about going back to the gym or dieting to loose a couple pounds. If she goes into postpartum depression she will gain even more weight.
As a guy that has been happily married for most of 21 years let me tell you her body will never be the same as the day you met her or any other day for the rest of your life. Her weight will cycle up and down and sometimes it is for reasons out of their control. I will say though it seems the happier I keep her the slimmer she stays. When she gets depressed is when the weight comes on and it can take moving a mountain to get it going the other direction. Telling her to diet or go to the gym only amplifies the depression.