(Reuters) - The U.S. Supreme Court’s key rulings on abortion, dating to the landmark 1973 decision that legalized the procedure nationwide, have long provoked social, religious and political tensions. As the court majority became more conservative over the decades, it set standards that enhance states’ ability to regulate abortion. Yet even in the most recent rulings, extending to 2007, there have never been five votes on the nine-member court to reverse the fundamental right to abortion. Here is a look at the court’s major abortion rulings. Roe v. Wade, 1973, 7-2 vote The court rules that a woman’s constitutional right to privacy protects her decision to end a pregnancy, and only a compelling state interest can justify regulating abortion. The decision institutes a three-part legal test, connected to a nine-month pregnancy and a fetus’ viability, that gives states more latitude for regulation with each trimester. Planned Parenthood of Southeastern Pennsylvania v. Casey, 1992, 5-4 vote The court affirms the right to abortion. Rather than the trimester framework, the test is whether a government regulation imposes an “undue burden” on a woman seeking an abortion by creating a “substantial obstacle” to the procedure before the fetus develops to the point at which it becomes viable. Stenberg v. Carhart, 2000, 5-4 vote The court decides that a Nebraska ban on a late-term procedure critics call “partial birth abortion” violates a woman’s right to abortion. The procedure, known medically as “intact dilation and extraction,” involves a physician removing most of the fetus intact rather than dismembering it. The justices fault the Nebraska ban, similar to laws in a majority of the states at the time, for its ambiguity about precisely what type of medical procedure is prohibited and for lacking an exception for the health of the mother. Gonzales v. Carhart, 2007, 5-4 vote The court decides that a federal Partial Birth Abortion Act, involving the same type of procedure as in the Nebraska dispute and without an exception for the woman’s health, is constitutional. The majority distinguishes the ruling from the 2000 case by noting that Congress developed findings that the procedure would never be necessary for the health of the mother. Another factor in the reversed course was that in 2000, Justice Sandra Day O’Connor had voted to strike down the partial-birth ban. She was succeeded by Samuel Alito in 2006, and in 2007 he cast the fifth vote in favor of the ban. Whole Woman’s Health v. Hellerstedt, 2016, 5-3 vote The court strikes down a Texas abortion law imposing strict regulations on doctors and facilities, finding that the 2013 statute put an undue burden on women exercising their right under the U.S. Constitution to end a pregnancy. The court declares that both provisions of the law - requiring physicians who perform abortions to have difficult-to-obtain “admitting privileges” at a local hospital and requiring clinics to install costly hospital-grade facilities - violate abortion rights. The ruling marked the firmest endorsement of U.S. abortion rights since the 1992 Planned Parenthood of Southeastern Pennsylvania v. Casey ruling. 