Can someone explain to me why this isn’t an obviously bad idea?  During pregnancy, quite a lot of the weight gain is blood volume, water retention, and the fetus plus the architecture that supports it; it’s not comparable to a non-pregnant 15 pound weight gain.

So if someone’s goal is to gain no weight during pregnancy, that’s going to amount to losing things like fat and bone density and muscle tissue quickly because there is no way to make a weightless placenta or a weightless baby, and doing this while expending all of the energy required to put together a little human being.  Fat doesn’t turn into a baby any more than fat turns into muscle while you exercise.