I find it depends on the sim. I've had Sims that their bodies "snap right back" so to speak after each pregnancy, I have Sims who become slightly curvier after a pregnancy but overall maintain their weight and I have had Sims who gain a lot with each pregnancy and have a really hard time shedding the extra weight and some never do. In one of my current saves Morgan Fyers fits this. She's had 3 kids and has become quite chubby despite gym trips and frequent jogs between pregnancies. Not that it matters, her hubby is crazy about her anyway, and is frankly developing a bit of a spread himself these days. :)
