Opponents have mounted a series of legal challenges to a new United States ban on late-term abortions.
Abortion is one of the most emotive issues in US politics
The ban - the first major limit on abortion in the US for 30 years - is being seen as a major victory for US conservatives.
Signing the bill, President George W Bush said the government had "come to the defence of the innocent child."
But critics are furious. Abortion rights groups are already taking legal action in California and New York.
And a federal judge in Nebraska blocked the law from applying to four doctors who filed suit against it - and against anyone they work with or who gets a referral from them.
US District Judge Richard Kopf said the law was "highly suspect, if not a per se violation of the Constitution" because it did not provide an exception to protect the health of the mother.
The law prohibits what it calls "partial birth abortions" - which it defines as intentionally killing a partially delivered foetus after the woman is artificially induced to give birth.
It says that the procedure is never necessary for health reasons, but includes an exception "to save the life of a mother".
Congress approved the bill last month.
Any doctor found practising the proscribed method of abortion now faces two years in jail.
"For years, a terrible form of violence has been directed against children who are inches from birth, while the law looked the other way," the US president said on Wednesday.
"Today at last, the American people and our government have confronted the violence, and come to the defence of the innocent child."
But pro-choice activists have condemned the bill as being too broad in its language.
The Democratic presidential candidate Howard Dean - who trained as a doctor - called the ban a "dark day for American women".
Bill Clinton vetoed two similar bills when he was president.
Total ban feared
Not since the Supreme Court declared women had a constitutional right to abortion in 1973 has there been such a major change to America's abortion laws, the BBC's Rob Watson in Washington says.
Pro-choice campaigners say the bill could criminalise several safe and common procedures.
They fear the law will represent the first step in a larger campaign eventually to ban all abortions.
Supporters of the bill argue that it applies only to a procedure done late in pregnancy - and relatively rarely - and that the procedure is never necessary to protect the health of the mother.
The law comes at a time when the US abortion rate has been dropping.
In 1999, 21.4 procedures were carried out per 1,000 women, compared to a rate of 27.4 per 1,000 in 1990, according to figures from the US National Center for Health Statistics.