By Ronnie Cohen

SAN FRANCISCO, Nov 5 (Reuters) - A federal appeals court panel on Monday sharply questioned lawyers defending an Arizona law that bans late-term abortions starting at 20 weeks of pregnancy except in medical emergencies, which opponents say is the toughest in the United States.

In San Francisco, a three-judge panel of the 9th U.S. Circuit Court of Appeals heard the case after it blocked the Republican-backed Arizona law from going into effect earlier this year. Three abortion providers challenged the law in court.

The Arizona law bars doctors from performing abortions starting at 20 weeks of pregnancy, except in medical emergencies, and could send doctors who perform them to jail.

The American Civil Liberties Union, which is suing to stop the law, said it was more extreme than similar laws elsewhere, because the way Arizona measures gestation means it would bar abortions two weeks earlier than in other states.

Those states also set the limit at 20 weeks but have different ways to calculate gestation time. Arizona already bans abortions at the point of viability, when a fetus might survive outside the womb, generally at 23 to 24 weeks.

Judge Andrew Kleinfeld, a panel member appointed by former President George H.W. Bush, repeatedly expressed concern that the law might not afford women the opportunity to abort a fetus with birth defects in cases where the defects are not apparent until just before 20 weeks.

He also questioned the need to prohibit abortions at that stage of the pregnancy, especially for fetuses bound to develop "horrible birth defects."

"They're basically born into hell and then die," Kleinfeld said. "I don't see how the courts could act before viability" of the fetus.

"With due respect, that's the woman's problem," responded David Cole, Arizona's solicitor general. "She should have made that decision earlier."

William Montgomery, the attorney for Maricopa County in Arizona who also defended the law before the appeals panel, said new medical evidence showed a fetus has the capacity to feel pain during an abortion at 20 weeks of development.

But Judge Marsha Siegel Berzon called that a "red herring" in terms of the constitutional questions the law raises.

The three-judge panel did not say when it could make a final ruling in the case. The U.S. Supreme Court legalized abortion nationwide in 1973, but has allowed states to place restrictions on the procedure from the time of viability unless the woman's health was at risk.

In July, days before the 9th Circuit panel blocked the law until it could fully consider the case, U.S. District Judge James Teilborg ruled that the Arizona measure was consistent with limits federal courts have allowed.

Talcott Camp, deputy director of the ACLU reproductive freedom project, said the Arizona law's exception to allow late-term abortion applies only in immediate emergencies if delay can jeopardize a woman's life or seriously harm her health.

"The medical emergency exception is truly, horrifically narrow," she said in a phone interview. "This is a law that allows her to get an abortion only when she is in emergency crisis."

Aside from Arizona, seven U.S. states have put laws into effect in the past two years banning late-term abortions, based on hotly debated medical research suggesting a fetus feels pain starting at 20 weeks of gestation, according to the ACLU.

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