The Bush administration argued in defense of the law, the , saying fetuses feel pain during such "inhumane" procedures.
The law, signed in November by President Bush, has not been enforced because judges in New York, Lincoln, Neb., and San Francisco agreed to hear evidence in three separate trials before deciding whether it violates the Constitution.
"These cases are all about determining whether [the law] is constitutional or not, to determine whether it ought to be enforced," said CBS News Legal Analyst Andrew Cohen. "Either these judges are going to rule it unconstitutional and it's going to go up the appellate ladder and it won't be enforceable for a little bit or they're going to rule it constitutional and it's going to be appealed. I think we're going to be in the courts awhile on this."
The law is the first substantial limitation on abortion since the Supreme Court's landmark Roe v. Wade decision. The current cases also appear likely to reach the high court.
Attorney A. Stephen Hut Jr., speaking for the plaintiffs, argued that the law "in its stunning breadth would ... remove the range of abortion alternatives available to women in the second trimester." He cautioned that the evidence will include "very raw stuff" and that descriptions of surgery were "not for the faint of heart."
Assistant U.S. Attorney Sean H. Lane described the law as an attempt to ban an "inhumane and gruesome procedure that causes pain to the fetus."
"Evidence at trial will illuminate that partial-birth abortion is never medically necessary and is an inhumane procedure that should be banned," Lane said in front of U.S. District Judge Richard C. Casey in Manhattan.
He said the law was specific in banning an abortion procedure that kills a "partially born fetus just inches from birth."
The lawsuits center on what opponents call "partial-birth" abortion and what doctors call "intact dilation and extraction."
In the procedure, generally performed in the second trimester and occasionally in the third, a fetus is partially delivered and its skull is punctured. An estimated 2,200 to 5,000 such abortions are performed annually in the United States, out of 1.3 million total abortions.
In Nebraska, a judge said he was troubled by an apparent lack of research done by Congress before passing the law.
"I have to tell you, I don't see that Congress spent much time" adequately looking at the issue, U.S. District Judge Richard Kopf said. "What deference then do I give Congress and its findings?"
Anthony Coppolino, who is presenting the Justice Department's case, said Congress used expert witnesses in formulating the law. "They had before them experts in the field," he said.
In San Francisco, a chief medical officer for Planned Parenthood testified that she chooses methods of abortion that violate the new law because they are among the safest options.
Asked by a government lawyer whether the fetus exhibits pain during the procedures, Maureen Paul replied, "I have no idea what you mean."
Abortion-rights groups argue the law was written too broadly and vaguely, and in such a way that threatens the health of some mothers. They say its language could criminalize more common types of abortion and could be a step toward abolishing abortion.
Supporters of the ban contend it applies only to a procedure done late in pregnancy that is never necessary to protect the health of the mother.
The Partial-Birth Abortion Act carries a maximum two-year prison term for doctors convicted of performing the procedure.
The Supreme Court struck down a similar Nebraska law almost four years ago because it lacked an exception for procedures done to preserve a woman's health. Anticipating this problem, Congress declared that "a partial birth abortion is never necessary to preserve the health of a woman" and is "outside the standard of medical care."
The abortion-rights groups say doctors may find themselves with no good alternative to the banned procedure to protect a woman's life or health if problems develop.
The American Medical Association does not encourage use of the procedure but says it should not be banned. The College of Obstetricians and Gynecologists says alternatives to the procedure usually exist, but in some circumstances it may be the best option.