Is it permissible to undergo surgery to tighten sagging abdominal skin after childbirth?

If the breast ptosis is minor and normal, and the purpose of the surgery is purely cosmetic, then it is not permissible to perform it; because it may be considered an alteration of Allah's creation. However, if the ptosis is severe, harmful, or detrimental, then there is no objection to performing the surgery to remove it.