It seems kids who are fat at birth are likely to be fat when they are older. This study showed that if a pregnant woman exercises on stationary bike, the baby's birth weight is lower, without interfering with maternal insulin resistance, which affects nutrient availability to the fetus.
I don't know. So far, they haven't shown that reducing a newborn's weight will have a life-long effect. It seems to me just as likely that the genes that make a baby fat at birth might make it fat later, and there doesn't seem to be a reason to believe that putting it on a diet in utero will affect that. I've dieted lots of times, with and without exercise, and my body always returns to the weight it wants to be.
And they say they reduce the size of the baby without affecting the system that makes nutrients available to the fetus. That's quite a trick.