(NewsNation Now) — Abortion rights have come up multiple times before the Supreme Court, but two cases have defined the United States’ stance on abortion: Roe v. Wade and Planned Parenthood v. Casey.

The 1973 Roe decision implemented a tier system for allowing safe abortion access. Tier One covers the first trimester of pregnancy and only requires basic safeguards for health. Limits on abortion access are not allowed.

Tier Two goes from the first trimester to the point of “fetal viability” or when the child can survive outside the uterus. That point is roughly about 24 weeks into the pregnancy. The state can only limit abortions to protect the mother’s health.

Tier Three goes from fetal viability until birth. In this tier, the state can restrict abortion as long as procedures are allowed when the mother’s life is at risk.

More than 90% of abortions are performed in the first 13 weeks of pregnancy, well before viability, according to the Centers for Disease Control and Prevention.

The 1992 Casey ruling reaffirmed Roe, but did away with the framework based around trimesters and instead created an “undue burden standard.”

States can now regulate abortion as well as limit access after the first trimester as long as the laws didn’t impose a substantial obstacle to prevent a woman from getting access.

Since Casey, cases have come before the court involving regulations around abortion access, but the undue burden standard case has been upheld each time.

The Supreme Court Wednesday will weigh whether to uphold a Mississippi law that bans abortion after 15 weeks and overrules Roe. Mississippi is also asking the court to overrule Casey, which reaffirmed Roe.

The court had never agreed to hear a case over an abortion ban so early in pregnancy until all three judges appointed by former President Donald Trump— Justices Neil Gorsuch, Brett Kavanaugh and Amy Coney Barrett — were on board.

A month ago, the justices also heard arguments over a uniquely designed Texas law that has succeeded in getting around the Roe and Casey decisions and banning abortions in the nation’s second-largest state after about six weeks of pregnancy. The dispute over the Texas law revolves around whether the law can be challenged in federal court, rather than the right to an abortion.

Despite its unusually quick consideration of the issue, the court has yet to rule on the Texas law, and the justices have refused to put the law on hold while the matter is under legal review.