On January 22, 1973, the US Supreme Court legalized abortion further to its decision in Roe v. Wade. In its decision, the Supreme Court layed out the foundation for the "trimester analysis" whereby in the first three months of pregnancy, no government regulation can restrict abortion; in the following trimester, the government may regulate only for the purpose of protecting women's health and preserving her safety; and last, allowing the government to fully regulate and ban abortions in the final trimester of the pregnancy. This decision was a crucial at the time as many women died every year in trying to get abortion through channels other than clinical as a result of the prohibition imposed.
Roe v. Wade was a heavy blow to the anti-abortionist groups who have attempted, in the following years, to have it overthrown but with little success. CNN's article discussing such subsequent events is very enlighting and I would recommend that you read through it. You can visit the below sites for more:
CNN's "35 years after Roe: A legacy of law and morality":
ROE v. WADE, 410 U.S. 113 (1973) - U.S. Supreme Court :
R. v. MORGENTALER  1 S.C.R.: