We've seen Heidi Klum go from radiantly pregnant to runway ready four times. The supermodel and reality host credits light cardio workouts and circuit training with celebrity trainer David Kirsch with getting her body back in shape post-baby. Kirsch says the biggest challenge women face after pregnancy is getting moving again. “For a woman who didn’t work out before or during the pregnancy, this is going to be a bigger challenge because she is going to have to find her discipline,” says Kirsch. So stay active throughout your pregnancy (with your doctor’s okay) for an easier transition to your post-pregnancy workouts.