Originally Published: 01 DEC 21 00:02 ET

By Joan Biskupic, CNN legal analyst & Supreme Court biographer

(CNN) -- When the Supreme Court hears a constitutional challenge on Wednesday to a Mississippi ban on abortion after 15 weeks of pregnancy, the justices will start with the two seminal decisions that secured the abortion right for women.

The decisions, in 1973 and 1992, laid down constitutional markers while describing in powerful terms the difficult issues at hand. In Roe v. Wade, the justices acknowledged at the outset "the sensitive and emotional nature of the abortion controversy, of the vigorous opposing views, even among physicians, and of the deep and seemingly absolute convictions that the subject inspires."

Nearly two decades later, in Planned Parenthood v. Casey, the justices asserted: "Men and women of good conscience can disagree, and we suppose some always shall disagree, about the profound moral and spiritual implications of terminating a pregnancy, even in its earliest stage. Some of us as individuals find abortion offensive to our most basic principles of morality, but that cannot control our decision. Our obligation is to define the liberty of all, not to mandate our own moral code."

At the heart of the Roe decision was protection for the woman's abortion decision before viability (when a fetus can survive outside the womb), which the justices estimated in 1973 at about 28 weeks. The court in 1992 said advances in neonatal care had brought that down to about 23 weeks of pregnancy, which is roughly where physicians draw the line today. The disputed Mississippi ban would prohibit abortion after 15 weeks -- plainly shattering the viability line.

The Casey decision affirmed the core abortion right and underscored adherence to precedent, the concept known as "stare decisis."

The court said in 1992 that retaining precedent was especially important for cases that, despite being politically divisive, had not been altered by new facts or changes in law. In Planned Parenthood v. Casey, the justices held fast to Roe's cutoff line of viability, saying that "viability marks the earliest point at which the State's interest in fetal life is constitutionally adequate to justify a legislative ban on nontherapeutic abortions."

As the remade Supreme Court, with three new appointees of former President Donald Trump, considers the fate of Roe v. Wade, it will inevitably weigh the principles of Casey, when the justices highlighted institutional integrity and legitimacy.

Here are the key holdings of those major decisions and the rationale that undergirded them:

Roe v. Wade: The trimester framework

Roe v. Wade was decided on January 22, 1973, by a 7-2 vote. It was a product of a time when justices defied partisan lineups. Justice Harry Blackmun (an appointee of President Richard Nixon) wrote the opinion. The only dissenters were Justices Byron White (an appointee of President John F. Kennedy) and William Rehnquist (a Nixon appointee).

As the court struck down a Texas abortion ban, it declared that the 14th Amendment's due process clause, which protects a right to privacy, covers a woman's right to end a pregnancy before fetal viability.

The justices established a trimester framework as they balanced a woman's interest with the state's: For the first trimester (roughly the first three months), the court said the abortion decision should be left to the woman and her physician; for the second trimester, a state could regulate the abortion procedure in ways reasonably related to the woman's health; for the final trimester, after fetal viability, the state could promote its "important and legitimate interest in potential life" and ban abortion except when necessary for the woman's life or health.

The justices acknowledged that the Constitution contains no explicit reference to a right of privacy but said that in a line of decisions dating to the late 1800s, "the Court has recognized that a right of personal privacy, or a guarantee of certain areas or zones of privacy, does exist under the Constitution. ... These decisions make it clear that only personal rights that can be deemed 'fundamental' or 'implicit in the concept of ordered liberty,' are included in this guarantee of personal privacy."

The Roe court said the right plainly extends to activities related to marriage, contraception and child rearing, and "is broad enough to encompass a woman's decision whether or not to terminate her pregnancy."

But the right is not absolute, the justices declared, referring to state interests "in safeguarding health, in maintaining medical standards, and in protecting potential life."

As the court established the viability cutoff point, it explained, "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 mother's womb. State regulation protective of fetal life after viability thus has both logical and biological justifications. If the State is interested in protecting fetal life after viability, it may go so far as to proscribe abortion during that period, except when it is necessary to preserve the life or health of the mother."

The Roe v. Wade decision followed two sets of oral arguments and months of difficult behind-the-scenes negotiations. On the first round, two justices had recently become ill and left the court and their successors were not yet in place, so only seven justices were on the bench. Given the weightiness of the question, the court decided to subject the case to re-arguments with all nine.

"One's philosophy, one's experiences, one's exposure to the raw edges of human existence, one's religious training, one's attitudes toward life and family and their values, and the moral standards one establishes and seeks to observe, are all likely to influence and to color one's thinking and conclusions about abortion," Blackmun wrote for the majority.

He added, "Our task, of course, is to resolve the issue by constitutional measurement, free of emotion and of predilection."

Planned Parenthood v. Casey's new standard: 'Undue burden'

Planned Parenthood of Southeastern Pennsylvania v. Casey was decided on June 29, 1992, by a 5-4 vote to affirm the central holding of Roe v. Wade giving women a right to end pregnancies before viability.

Justices Sandra Day O'Connor, Anthony Kennedy and David Souter joined forces to write the opinion. Rehnquist, White and Justices Antonin Scalia and Clarence Thomas dissented. A part of the decision, upholding specific Pennsylvania abortion restrictions, came down to a separate 7-2 vote, from which Blackmun and Justice John Paul Stevens dissented. (The only remaining member of that 1992 court is Thomas.)

As in 1973, the bench was less defined by party politics. The only one of the nine who had not been appointed by a Republican president was White.

The majority reaffirmed Roe based on principles of "stare decisis," that is, adherence to precedent. The court reiterated that women have a right to abortion before fetal viability. It also reasserted that a state may restrict abortions after viability, if the law contains exceptions for a woman's life or health.

The court, however, discarded the trimester approach and diluted the standard for when a state regulation violates the abortion right. The court said the trimester framework had failed to sufficiently accommodate state interests and that the new standard should be whether a regulation puts an "undue burden" on a woman seeking an abortion.

"A finding of an undue burden is a shorthand for the conclusion that a state regulation has the purpose or effect of placing a substantial obstacle in the path of a woman seeking an abortion of a nonviable fetus," the plurality wrote.

"A statute with this purpose is invalid because the means chosen by the State to further the interest in potential life must be calculated to inform the woman's free choice, not hinder it," the justices added. "And a statute which, while furthering the interest in potential life or some other valid state interest, has the effect of placing a substantial obstacle in the path of a woman's choice cannot be considered a permissible means of serving its legitimate ends."

The justices in Planned Parenthood v. Casey again emphasized that the court had historically afforded constitutional protection to personal decisions relating to marriage and contraception.

Speaking to the practical importance of the Roe precedent, the justices emphasized that people had relied on the decision for two decades at that point and added, "The ability of women to participate equally in the economic and social life of the Nation has been facilitated by their ability to control their reproductive lives."

Considering the court's own integrity, the justices said overruling Roe "would seriously weaken the Court's capacity to exercise the judicial power and to function as the Supreme Court of a Nation dedicated to the rule of law."

The justices warned in 1992 against "a surrender to political pressure" and insisted that reversal of a watershed decision "in the absence of the most compelling reason ... would subvert the Court's legitimacy beyond any serious question."