with whom The Chief Justice and Me. Justice Rehnquist join, dissenting.
Because the Court now withdraws from the States a substantial measure of the power to protect fetal life that was reserved to them in Roe v. Wade, 410 U. S. 113 (1973), and reaffirmed in Planned Parenthood of Central Missouri v. Danforth, 428 U. S. 52 (1976), I file this dissent.
I
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 *402life 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 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) (emphasisadded). 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 *403believed 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.
In the original opinion and judgment of the three-judge court, Planned Parenthood Assn. v. Fitzpatrick, 401 F. Supp. 554 (ED Pa. 1975), § 5 (a) was invalidated on two grounds: first, because it required a determination of viability and because that term, as defined in § 2, was held to be unen-forceably vague; and second, because the section required a determination of when a fetus may be viable, it was thought to regulate a period of time prior to viability and was therefore considered to be invalid under this Court’s cases. The District Court was not disturbed by the fact that its opinion declared the term “viability” as used in this Court’s opinion in Roe v. Wade to be hopelessly vague since it understood that opinion also to have given specific content to that term and to have held that a State could not consider any fetus to be viable prior to the 24th week of pregnancy. This was concrete guidance to the States, and because the “may be viable” provision of § 5 (a) “tend[ed] to carve out a . . . period of time of potential viability [which might cover a period of] 20 to 26 weeks gestation,” 401 F. Supp., at 572, the State was unlawfully regulating the second trimester. Because it sought to enforce § 5 (a), § 5 (d) was also invalidated. Section 6 (b), which forbade all abortions after viability, also fell to the challenge of vagueness.
The District Court’s judgment was pending on appeal here when Planned Parenthood of Central Missouri v. Danforth, *404supra, was argued and decided. There, the state Act defined viability as “that stage of fetal development when the life of the unborn child may be continued indefinitely outside the womb by natural or artificial life-supportive systems.” 428 U. S., at 63. This definition was attacked as impermis-sibly expanding the Roe v. Wade definition of viability; the “mere possibility of momentary survival,” it was argued, was not the proper standard under the Court's cases. 428 U. S., at 63. It was also argued in this Court that the “may be” language of the Missouri statute was vulnerable for the same reasons that the “may be” provision of the Pennsylvania statute had been invalidated by the District Court in the case now before us. Brief for Appellants, O. T. 1975, No. 74M151, pp. 65-66, quoting Planned Parenthood Assn. v. Fitzpatrick, supra, at 571-572. This Court, however, rejected these arguments and sustained the Missouri definition as consistent with Roe, “even when read in conjunction with” another section of the Act that proscribed all abortions not necessary to preserve the life or health of the mother “unless the attending physician first certifies with reasonable medical certainty that the fetus is not viable,” that is, that it has not reached that stage at which it may exist indefinitely outside the mother’s womb. 428 U. S., at 63-64. The Court noted that one of the appellant doctors “had no particular difficulty with the statutory definition” and added that the Missouri definition might well be considered more favorable to the complainants than the Roe definition since the “point when life can be ‘continued indefinitely outside the womb’ may well occur later in pregnancy than the point where the fetus is ‘potentially able to live outside the mother’s womb.’ ” 428 U. S., at 64. The Court went on to make clear that it was not the proper function of the legislature or of the courts to place viability at a specific point in the gestation period. The “flexibility of the term,” which was essentially a medical concept, was to be preserved. Ibid. The Court plainly reaffirmed what it had held *405in Roe v. Wade: Viability refers not only to that stage of development when the fetus actually has the capability of existing outside the womb but also to that stage when the fetus may have the ability to do so. The Court also reaffirmed that at any time after viability, as so understood, the State has the power to prohibit abortions except when necessary to preserve the life or health of the mother.
In light of Danforth, several aspects of the District Court's judgment in the Fitzpatrick case were highly questionable, and that judgment was accordingly vacated and remanded to the District Court for reconsideration. Beal v. Franklin, 428 U. S. 901 (1976). A drastically modified judgment eventuated. The term “viability” could not be deemed vague in itself, and hence the definition of that term in § 2 and the proscription of § 6 (b) against post-viability abortions were sustained. The District Court, however, in a conclusory opinion adhered to its prior view that § 5 (a) was unconstitutional, as was § 5 (d) insofar as it related to § 5 (a).
Affirmance of the District Court’s judgment is untenable. The District Court originally thought § 5 (a) was vague because the term “viability” was itself vague. The Court scotched that notion in Danforth, and the District Court then sustained the Pennsylvania definition of viability. In doing so, it necessarily nullified the major reason for its prior invalidation of § 5 (a), which was that it incorporated the supposedly vague standard of § 2. But the District Court had also said that the “may be viable” standard was invalid as an impermissible effort to regulate a period of “potential” viability. This was the sole remaining articulated ground for invalidating § 5 (a). But this is the very ground that was urged and rejected in Danforth, where this Court sustained the Missouri provision defining viability as the stage at which the fetus “may” have the ability to survive outside the womb and reaffirmed the flexible concept of viability announced in Roe.
*406In affirming the District Court, the Court does not in so many words agree with the District Court but argues that it is too difficult to know whether the Pennsylvania Act simply intended, as the State urges, to go no further than Roe permitted in protecting a fetus that is potentially able to survive or whether it intended to carve out a protected period prior to viability as defined in Roe. The District Court, although otherwise seriously in error, had no such trouble with the Act. It understood the “may be viable” provision as an attempt to protect a period of potential life, precisely the kind of interest that Roe protected but which the District Court erroneously thought the State was not entitled to protect.1 Danforth, as I have said, reaffirmed Roe in this respect. Only those with unalterable determination to invalidate the Pennsylvania Act can draw any measurable difference insofar as vagueness is concerned between “viability” defined as the ability to survive and “viability” defined as that stage at which the fetus may have the ability to survive. It seems to me that, in affirming, the Court is tacitly disowning the “may be” standard of the Missouri law as well as the “potential ability” *407component of viability as that concept was described in Roe. This is a further constitutionally unwarranted intrusion upon the police powers of the States.
II
Apparently uneasy with its work, the Court has searched for and seized upon two additional reasons to support affirmance, neither of which was relied upon by the District Court. The Court first notes that under § 5 (d), failure to make the determinations required by § 5 (a), or otherwise to comply with its provisions, subjects the abortionist to criminal prosecution under those laws that “would pertain to him had the fetus been a child who was intended to be bom and not aborted.” Although concededly the Pennsylvania law of criminal homicide conditions guilt upon a finding that the defendant intentionally, knowingly, recklessly, or negligently caused the death of another human being, the Court nevertheless goes on to declare that the abortionist could be successfully prosecuted for criminal homicide without any such fault or omission in determining whether or not the fetus is viable or may be viable. This alleged lack of a scienter requirement, the Court says, fortifies its holding that § 5 (a) is void for vagueness.
This seems to me an incredible construction of the Pennsylvania statutes. The District Court suggested nothing of the sort, and appellees focus entirely on § 5 (a), ignoring the homicide statutes. The latter not only define the specified degrees of scienter that are required for the various homicides, but also provide that ignorance or mistake as to a matter of fact, for which there is a reasonable explanation, is a defense to a homicide charge if it negatives the mental state necessary for conviction. Pa. Stat. Ann., Tit. 18, § 304 (Purdon 1973). Given this background, I do not see how it can be seriously argued that a doctor who makes a good-faith mistake about whether a fetus is or is not viable could be successfully prose*408cuted for criminal homicide. This is the State’s submission in this Court; the court below did not address the matter; and at the very least this is something the Court should not decide without hearing from the Pennsylvania courts.
Secondly, the Court proceeds to find the standard-of-care provision in § 5 (a) to be impermissibly vague, particularly because of an asserted lack of a mens rea requirement. I am unable to agree. In the first place, the District Court found fault with § 5 (a) only because of its viability and “may be viable” provisions. It neither considered nor invalidated the standard-of-care provision. Furthermore, the complaint did not expressly attack § 5 (a) on this ground, and plaintiffs’ request for findings and conclusions challenged the section only on the grounds of the overbreadth and vagueness of the viability and the “may be viable” provisions. There was no request to invalidate the standard-of-care provision. Also, the plaintiffs’ post-trial brief dealt with the matter in only the most tangential way. Appellees took no cross-appeal; and although they argue the matter in their brief on the merits in this Court, I question whether they are entitled to have still another provision of the Pennsylvania Act declared unconstitutional in this Court in the first instance, thereby and to that extent expanding the relief they obtained in the court below.2 United States v. New York Telephone Co., 434 U. S. 159, 166 n. 8 (1977).
In any event, I cannot join the Court in its determined attack on the Pennsylvania statute. As in the case with a mistaken viability determination under § 5 (a), there is no basis for asserting the lack of a scienter requirement in a prosecution for violating the standard-of-care provision. I agree with the State that there is not the remotest chance that any abortionist will be prosecuted on the basis of a good-*409faith mistake regarding whether to abort, and if he does, with respect to which abortion technique is to be used. If there is substantial doubt about this, the Court should not complain of a lack of an authoritative state construction, as it does, but should direct abstention and permit the state courts to address the issues in the light of the Pennsylvania homicide laws with which those courts are so much more familiar than are we or any other federal court.
Ill
Although it seems to me that the Court has considerably narrowed the scope of the power to forbid and regulate abortions that the States could reasonably have expected to enjoy under Roe and Danforth, the Court has not yet invalidated a statute simply requiring abortionists to determine whether a fetus is viable and forbidding the abortion of a viable fetus except where necessary to save the life or health of the mother. Nor has it yet ruled that the abortionist’s determination of viability under such a standard must be final and is immune to civil or criminal attack. Sections 2 and 6 (b) of the Pennsylvania law, for example, remain undisturbed by the District Court’s judgment or by the judgment of this Court.
What the Court has done is to issue a warning to the States, in the name of vagueness, that they should not attempt to forbid or regulate abortions when there is a chance for the survival of the fetus, but it is not sufficiently large that the abortionist considers the fetus to be viable. This edict has no constitutional warrant, and I cannot join it.
The District Court observed:
“Roe makes it abundantly clear that the compelling point at which a state in the interest of fetal life may regulate, or even prohibit, abortion is not before the 24th week of gestation of the fetus, at which point the Supreme Court recognized the fetús then presumably has the capability of meaningful life outside the mother’s womb. Consequently, Roe recognizes only two periods concerning fetuses. The period prior to viability, when the state may not regulate in the interest of fetal life, and the period after viability, when it may prohibit altogether or regulate as it sees fit. The ‘may be viable’ provision of Section 5 (a) tends to carve out a third period of time of potential viability.” Planned Parenthood Assn. v. Fitzpatrick, 401 F. Supp. 654, 572 (ED Pa. 1975) (emphasis added). Thus, the court interpreted the term “viability” more restrictively than Roe, read in its entirety, permitted but coextensively with the definition in § 2. Based on its misapprehension of Roe, the court condemned § 5 (a) essentially for reaching the period when the fetus has the potential “capability of meaningful life outside the mother’s womb.” Ibid.
Unquestionably, rehabilitating § 5 (a) to satisfy this Court’s opinion will be a far more extensive and more difficult task than that which the State faced under the District Court’s ruling.