As the justices consider the Mississippi law, they will have to take account of the controlling opinion in Planned Parenthood v. Casey, a joint effort by three Republican appointees, Justices Sandra Day O’Connor, David H. Souter and Anthony M. Kennedy. That ruling affirmed what it called Roe v. Wade’s essential holding, that states may not prohibit abortions before fetal viability.

Casey also introduced a more relaxed standard to evaluate restrictions short of outright bans before fetal viability, saying that states could not impose an undue burden on the right to 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 opinion said.

“Our adoption of the undue burden analysis does not disturb the central holding of Roe v. Wade and we reaffirm that holding,” it continued. “Regardless of whether exceptions are made for particular circumstances, a state may not prohibit any woman from making the ultimate decision to terminate her pregnancy before viability.”