The case was a seminal one in the history of abortion rights in the United States. It was the first case which provided an opportunity to overturn Roe since the two most liberal Justices, William Brennan and Thurgood Marshall, were replaced with the Bush-appointed Justices David Souter and Clarence Thomas. Both were viewed as ostensible conservatives compared to their predecessors. This left the Court with eight Republican-appointed justices - five of whom had been appointed by Presidents Reagan or Bush, both of which were well known for their opposition to Roe. Finally, the only remaining Democratic appointee - Justice Byron White - had been one of the two dissenters from the original Roe decision.
At this point, only two of the Justices were obvious supporters of Roe v. Wade: Blackmun, the author of Roe, and Stevens, who had joined opinions specifically reaffirming Roe in City of Akron v. Akron Center for Reproductive Health and Thornburgh v. American College of Obstetricians and Gynecologists. Given these circumstances, even most pro-choice advocates expected Roe to be overruled and were gearing up for a subsequent state-by-state campaign against the passage of particular anti-abortion laws
The case was argued by ACLU attorney Kathryn Kolbert for Planned Parenthood. Pennsylvania attorney general Ernest Preate, Jr. argued the case for the State. In the Supreme Court oral arguments, Solicitor General Kenneth Starr spoke for the Bush Administration.
At the conference of the Justices two days after oral argument, Justice Souter defied expectations and voted against any gutting of Roe v. Wade, joining Justices O'Connor, Stevens, and Blackmun, who had likewise refused to do so three years earlier in Webster v. Reproductive Health Services. This resulted in a precarious five Justice majority consisting of Chief Justice William Rehnquist, Byron White, Antonin Scalia, Anthony Kennedy, and Clarence Thomas that favored upholding all the abortion restrictions and in effect reducing Roe to dead letter. However, Kennedy changed his mind shortly thereafter and secretly joined with fellow Reagan-Bush justices Sandra Day O'Connor and David Souter to write a plurality opinion that would reaffirm Roe.
Except for three opening sections of the O'Connor-Kennedy-Souter opinion, Casey a divided judgment, as no other sections of any opinion were joined by a majority of justices. However, the plurality decision jointly written by Justices Souter, O'Connor, and Kennedy is recognized as the lead opinion with precedential weight because each of its parts were concurred in by at least two other Justices, albeit different ones for each part.
These three justices began their written opinion by noting the U.S. government's previous challenges to Roe v. Wade:
The plurality opinion stated that it was upholding what it called the "essential holding" of Roe. The plurality asserted that the right to abortion is grounded in the Due Process Clause of the Fourteenth Amendment, and the plurality reiterated what the Court had said in Eisenstadt v. Baird: "[i]f the right of privacy means anything, it is the right of the individual, married or single, to be free from unwarranted governmental intrusion into matters so fundamentally affecting a person as the decision whether to bear or beget a child."
The plurality's opinion also included some controversial language about the doctrine of stare decisis. The plurality emphasized the need to stand by prior decisions even if they were unpopular, unless there had been a change in the fundamental reasoning underpinning the previous decision. It also acknowledged the need for predictability and constancy in judicial decision making. For example,
The plurality went on to give society's rejection of the "Separate but Equal" concept as a legitimate reason for the Brown v. Board of Education court’s rejection of the Plessy v. Ferguson doctrine. Emphasizing the need to not be seen as overruling a prior decision merely because the individual members of the Court had changed, O’Connor states,
Since the plurality overruled some portions of Roe v. Wade despite its emphasis on stare decisis, Chief Justice Rehnquist in dissent argued that this section was entirely obiter dicta. All these opening sections were joined by Justices Blackmun and Stevens for the majority. The remainder of the decision did not command a majority, but at least two other Justices concurred in judgment on each of the remaining points.
The plurality then overturned the strict trimester formula used in Roe to weigh the woman's interest in obtaining an abortion against the State's interest in the life of the fetus. Continuing advancements in medical technology meant that at the time Casey was decided, a fetus might be considered viable at 22 or 23 weeks rather than at the 28 weeks that was more common at the time of Roe. The plurality recognized viability as the point at which the state interest in the life of the fetus outweighs the rights of the woman and abortion may be banned entirely.
The plurality also replaced the heightened scrutiny of abortion regulations under Roe, which was standard for fundamental rights in the Court's case law, with a lesser "undue burden" standard previously developed by O'Connor in her dissent in Akron v. Akron Center for Reproductive Health. A legal restriction posing an undue burden was defined as one having "the purpose or effect of placing a substantial obstacle in the path of a woman seeking an abortion of a nonviable fetus." The plurality also overruled Akron v. Akron Center for Reproductive Health, 462 U.S. 416 (1983) and Thornburgh v. American College of Obstetricians and Gynecologists, 476 U.S. 747 (1986) , each of which applied "strict scrutiny" to abortion restrictions.
Applying this new standard to the Pennsylvania Act under challenge, the plurality struck down the spousal notification requirement, stating that it gave too much power to husbands over their wives and would worsen situations of spousal abuse. The plurality upheld the State's 24 hour waiting period, informed consent, and parental notification requirements, holding that none constituted an undue burden.
William Rehnquist, Byron White, Harry Blackmun, John Paul Stevens, Antonin Scalia, and Clarence Thomas - the six Justices who did not join the plurality opinion - wrote or joined opinions in which they partially concurred and partially dissented from the decision.
Rehnquist and Scalia each joined the plurality in upholding the parental consent, informed consent, and waiting period laws. However, they dissented from the plurality's decision to uphold Roe v. Wade and strike down the spousal notification law, contending that Roe was incorrectly decided. Rehnquist and Scalia joined each other's concurrence/dissents, and White and Thomas, who did not write their own opinions, joined in both.
Blackmun and Stevens wrote opinions in which they approved of the plurality's preservation of Roe and rejection of the spousal notification law. They did not, however, agree with the plurality's decision to the other three laws at issue. Blackmun went further, sharply attacking and criticizing the anti-Roe bloc of the Court. Neither Blackmun's nor Stevens's opinions were joined by other justices.