Liposuction is a great tool to use to make changes in body contour. It physically removes fat cells from areas that are bigger and can even out the overall shape. When the fat is removed we have to rely on the elasticity of the skin to shrink to match the new contour underneath. If the skin has been stretched (by pregnancy or weight gain) as it often is in the midsection, it may not shrink enough and even sag more causing it to look worse with liposuction. Sometimes the correct procedure may not be liposuction to fix your issues. You may need removal of skin and tightening of muscles to actually get a better contour. This is achieved with a tummy tuck.
The best advice is if you are planning to lose some weight, do it before you have your liposuction. Even if your skin tone is good after weight loss, your surgeon will be able to get a better, longer lasting result by sculpting the fat starting from your slimmer contour. If you lose a significant amount of weight, chances are the skin will be loose and you will get a better result by having a tummy tuck that will address all of the issues rather than just the fat.