I think it depends on the Sim. I'm playing the 100 Baby Challenge. My first matriarch was super skinny before getting pregnant and probably by her 5th pregnancy she started to pack on the pounds. She had 13 kids in her generation....I got her dog, had her jog with the dog & she was able to lose some of the baby fat, but not all of it.
But then my 2nd matriarch, she didn't gain any weight after multiple pregnancies. She stayed thin and reverted back to her original size. I'm now on my 3rd matriarch and she actually gained weight after her 1st child. She's on kid 7, and is a bit on the heavy side.
I actually like how this is in game, as it's similar to real life. Every woman is different.
