"Because it is central to the operation of various other requirements, we begin with the statute's definition of medical emergency. Under the statute, a medical emergency is "[t]hat condition which, on the basis of the physician's good faith clinical judgment, so complicates the medical condition of a pregnant woman as to necessitate the immediate abortion of her pregnancy to avert her death or for which a delay will create serious risk of substantial and irreversible impairment of a major bodily function." 18 Pa.Cons.Stat. (1990). § 3203. Petitioners argue that the definition is too narrow, contending that it forecloses the possibility of an immediate abortion despite some significant health risks. If the contention were correct, we would be required to invalidate the restrictive operation of the provision, for the essential holding of Roe forbids a State from interfering with a woman's choice to undergo an abortion procedure if continuing her pregnancy would constitute a threat to her health. 410 U.S., at 164, 93 S.Ct., at 732. See also Harris v. McRae, 448 U.S., at 316, 100 S.Ct., at 2687."