Opinion of the Court
be expected from a legislative body.
iii
What Roe did not provide was any cogent justification for the lines it drew. Why, for example, does a State have no authority to regulate first trimester abortions for the purpose of protecting a woman's health? The Court's only explanation was that mortality rates for abortion at that stage were lower than the mortality rates for childbirth. Roe, 410 U. S., at 163. But the Court did not explain why mortality rates were the only factor that a State could legitimately consider. Many health and safety regulations aim to avoid adverse health consequences short of death. And the Court did not explain why it departed from the normal rule that courts defer to the judgments of legislatures "in areas fraught with medical and scientific uncertainties." Marshall v. United States, 414 U. S. 417, 427 (1974).
An even more glaring deficiency was Roe's failure to justify the critical distinction it drew between pre- and post-viability abortions. Here is the Court's entire explanation:
With respect to the State's important and legitimate interest in potential life, the "compelling" point is at viability. This is so because the fetus then presumably has the capability of meaningful life outside the womb.
Roe, 410 U. S., at 163.
As Professor Laurence Tribe has written, "[c]learly, this mistakes 'a definition for a syllogism.'" Tribe 4 (quoting Ely 924). The definition of a "viable" fetus is one that is capable of surviving outside the womb, but why is this the point at which the State's interest becomes compelling? If, as Roe held, a State's interest in protecting prenatal life is compelling "after viability," 410 U. S., at 163, why isn't that interest "equally compelling before viability"? Webster v. Reproductive Health Servs., 492 U. S. 490, 519 (1989) (plurality) (quoting Thornburgh v. American College of Obstetricians
