By a 7 to 2 vote in 1973, the Supreme Court established a constitutional right to abortion, striking down laws in many states that had barred the procedure. The court said states could not ban abortions before fetal viability, the point at which the fetus can survive outside the womb. That was around 28 weeks at the time and, because of improvements in medical technology, is around 23 weeks now.

Roe also established a framework to govern abortion regulation based on the trimesters of pregnancy. In the first trimester, it allowed almost no regulations. In the second, it allowed regulations to protect women’s health. In the third, it allowed states to ban abortions so long as exceptions were made to protect the life and health of the mother.

The court discarded the trimester framework in 1992 in Planned Parenthood v. Casey. But Casey retained what it called Roe’s “essential holding” — that women have a constitutional right to terminate their pregnancies until fetal viability.