To be honest I never read any books so it's not a recommended thing or anything, but I didn't want to lose my stomach muscles so just did that thing where you clench all the tummy muscles and let go whenever I thought of it. I sometimes worry that this kept her small (born at 35th percentile 3.25kg) but that's probably silly /unrelated. I did keep my tummy muscles and they survived the final month(s?) of mega stretch. But again this is a single data point not proof. It's a micro exercise habit I've had that I kept up.