"In Roe v. Wade, the Court defined the term "viability" to signify the stage at which a fetus is "potentially able to live outside the mother's womb, albeit with artificial aid." This is the point at which the State's interest in protecting fetal life becomes sufficiently strong to permit it to "go so far as to proscribe abortion during that period, except when it is necessary to preserve the life or health of the mother." 410 U.S. at 410 U. S. 163-164. The Court obviously crafted its definition of viability with some care, and it chose to define that term not as that stage of development at which the fetus actually is able or actually has the ability to survive outside the mother's womb, with or without artificial aid, but as that point at which the fetus is potentially able to survive. In the ordinary usage of these words, being able and being potentially able do not mean the same thing. Potential ability is not actual ability. It is ability "[e]xisting in possibility, not in actuality." Webster's New International Dictionary (2d ed.1958). The Court's definition of viability in Roe v. Wade reaches an earlier point in the development of the fetus than that stage at which a doctor could say with assurance that the fetus would survive outside the womb. It was against this background that the Pennsylvania statute at issue here was adopted, and the District Court's judgment was entered. Insofar as Roe v. Wade was concerned, Pennsylvania could have defined viability in the language of that case -- "potentially able to live outside the mother's womb" -- and could have forbidden all abortions after this stage of any pregnancy. The Pennsylvania Act, however, did not go so far. It forbade entirely only those abortions where the fetus had attained viability as defined in § 2 of the Act, that is, where the fetus had "the capability . . . to live outside the mother's womb albeit with artificial aid." Pa.Stat.Ann., Tit. 35, § 6602 (Purdon 1977) (emphasis added). But the State, understanding that it also had the power under Roe v. Wade to regulate where the fetus was only "potentially able" to exist outside the womb, also sought to regulate, but not forbid, abortions where there was sufficient reason to believe that the fetus "may be viable"; this language was reasonably believed by the State to be equivalent to what the Court meant in 1973 by the term "potentially able to live outside the mother's womb." Under § 5(a), abortionists must not only determine whether the fetus is viable but also whether there is sufficient reason to believe that the fetus may be viable. If either condition exists, the method of abortion is regulated, and a standard of care imposed. Under § 5(d), breach of these regulations exposes the abortionist to the civil and criminal penalties that would be applicable if a live birth, rather than an abortion, had been intended."