Balloons, you are hysterical! Most people know this information, so no need for a biology lesson. Your friend's husband doesn't seem to be the sharpest knife in the drawer.
Those of us who have children and grandchildren know that when a woman becomes pregnant, her body will change. Some, more drastically than others, according to genetics, etc. In some women, the changes remain. Some others seem to go right back to their lithe, pre-pregnancy figures. I don't know if there is a predictor of this, except for maybe looking at her mother, older sisters, and other female relatives. I do know this: once the woman becomes a mother, the world is no longer about you, Tarzan. It is now about the baby. You'll have to settle for 2nd place. Women do not like being ridiculed (even "playfully") about their bodies when they are pregnant or after they have the child. Making jokes about her figure, or chiding her into a gym is not funny. She may smile. She may laugh it off. But inside it hurts her very much. Your idyllic image of her may never return.
So, did you marry her because you loved her, or because she was "hot?" Is she your life mate, or just an ornament to wear on your arm?

However, there is one surefire way to motivate her to once again become that idyllic hottie that you so desire.
Keep doing this. Continue to destroy her emotionally.
She will eventually dump you for the idiot you are, THEN she will go to the gym or pick up some sport that gets her back in shape (divorce is a GREAT motivator). Just in time for the next guy to come along and discover your former "hottie." C'est la vie...

Men, think long and hard about marriage and what happens, especially when you have your first child. If the thought of her gaining a few pounds terrifies you, then you're better off staying single and calling a professional.

Love you, Balloons!