The five-to-four decision in a case challenging the constitutionality of a ban on what critics call "partial birth abortion" is a reflection of the impact of the recent addition to the US high court of two conservative justices appointed by George Bush, the US president.
Abortion opponents condemn the procedure as infanticide, because the head and torso of the foetus is delivered intact while alive, but the foetus does not die until doctors extract its brain.
Bush hailed the decision, which he said affirms the "culture of life" espoused by many Americans.
"The partial-birth abortion ban, which an overwhelming bipartisan majority in congress passed and I signed into law, represents a commitment to building a culture of life in America."
Bush also said that years-long efforts to outlaw the "abhorrent procedure" reflect "the compassion and humanity of America."
"The Supreme Court's decision is an affirmation of the progress we have made over the past six years in protecting human dignity and upholding the sanctity of life," he said.
"We will continue to work for the day when every child is welcomed in life and protected in law."
Outright ban
Justice Anthony Kennedy, writing for the high court majority, wrote that the ban does not impose an "undue burden" since there were other options, such as making sure the foetus was not intact when it had to be extracted.
US anti-abortion activists hailed the watershed ruling, and expressed hope that the decision eventually will pave the way to an outright ban on all abortions across the country.
"The time is now right to launch aggressive legal challenges across America to abortion on demand," said Troy Newmann, of the anti-abortion group Operation Rescue.
Unease
For their part, "pro-choice" interests said they were fearful about the future of access to abortion, although they couched their unease in the narrow terms of Wednesday's decision.
Justice Ruth Bader Ginsburg, in a sharply worded dissent, called Wednesday's decision "alarming."
"It tolerates, indeed applauds, federal intervention to ban nationwide a procedure found necessary and proper in certain cases," she said.
"For the first time since Roe, the Court blesses a prohibition with no exception safeguarding a woman's health," Ginsburg wrote.
US Senator Barbara Boxer, a pro-choice advocate, echoed that alarm.
"As a result of today's ruling, the health of women who have dangerous pregnancies is now in deep jeopardy," the California politician said.
"Women who are in need of this banned procedure will be denied it, even if they risk losing their fertility, becoming paralysed or sustaining organ damage" while having a less safe procedure, Boxer said.
According to estimates, around 10 per cent of the roughly 1.2 million annual voluntary abortions take place after the third month of pregnancy; the method in question affects a few thousand of those.