Tuesday marks the 40th anniversary of the Roe v. Wade Supreme Court decision. In a 7-2 ruling on January 22, 1973, the justices declared laws prohibiting abortion violated a woman’s constitutional right to privacy. They also said states could regulate abortion procedures in the interest of a woman’s health or in protecting a potential human life starting at the end of the pregnancy’s first trimester.

Abortion was legal under common law in the United States leading up to the 19th century, says Leslie Reagan, professor of history and law at the University of Illinois and author of “When Abortion was a Crime.” Early laws only prohibited the use of toxic substances to cause miscarriages after “quickening,” or when a woman feels her child move — usually four or five months into the pregnancy.

“That was the moral point where people understood there was a life,” Reagan says.

Since then, the definition of life has been debated many times over, but Roe v. Wade remains the law of the land.

Opinion: Millennials have the power to protect Roe v. Wade

Learn more about the events leading up to this historic decision, and what’s happened in the four decades since the ruling:

(Read More On CNN)