Would I need to lose weight (I weigh 256 pounds) before having fat transfer from my stomach and thighs to my buttocks?
You absolutely need to lose weight. At your present height and weight, you have a BMI of 37.5. This is significantly too high in the obese category and almost to morbidly obese. You should be no more than 170 pounds. This is dangerous to your health even without surgery and it definitely raises your risks during surgery. The other question is, why, when you are so overweight, would you want to add fat somewhere. Lose the weight. After you do, you may find that you no longer desire buttock augmentation. If you do not lose as much as you desire from the abdomen and thighs, you may, instead of fat transfer to the buttocks, simply need some liposuction of the thighs and abdomen.
R.T. Buchanan, MD