Section 12O. If an abortion is performed pursuant to section twelve M, no abortion procedure which is designed to destroy the life of the unborn child or injure the unborn child in its mother’s womb may be used unless, in the physician’s best medical judgment, all other available procedures would create a greater risk of death or serious bodily harm to the mother either at the time of the abortion, or subsequently as the result of a future pregnancy, than the one being used.