CHICAGO – A review of medical evidence has found that fetuses likely don’t feel pain until the final months of pregnancy, a powerful challenge to abortion opponents who hope that discussions about fetal pain will make women think twice about ending pregnancies.

Critics angrily disputed the findings and claimed the report is biased.

“They have literally stuck their hands into a hornet’s nest,” said Dr. Kanwaljeet Anand, a fetal pain researcher at the University of Arkansas for Medical Sciences, who believes fetuses as young as 20 weeks old feel pain. “This is going to inflame a lot of scientists who are very, very concerned and are far more knowledgeable in this area than the authors appear to be. This is not the last word – definitely not.”

The review by researchers at the University of California, San Francisco comes as advocates are pushing for fetal pain laws aimed at curtailing abortion. Proposed federal legislation would require doctors to provide fetal pain information to women seeking abortions when fetuses are at least 20 weeks old, and to offer women fetal anesthesia at that stage of the pregnancy. A handful of states have enacted similar measures.

But the report, appearing in today’s Journal of the American Medical Association, says that offering fetal pain relief during abortions in the fifth or sixth months of pregnancy is misguided and might result in unacceptable health risks to women.

Dr. Nancy Chescheir, chairman of obstetrics and gynecology at Vanderbilt University and a board director at the Society of Maternal-Fetal Medicine, said the article “will help to develop some consensus” on when fetuses feel pain. “To date, there hasn’t been any.”

The researchers reviewed dozens of studies and medical reports and said the data indicate that fetuses likely are incapable of feeling pain until around the seventh month of pregnancy, when they are about 28 weeks old.

While brain structures involved in feeling pain begin forming much earlier, research indicates they likely do not function until the pregnancy’s final stages, said the report’s senior author, UCSF obstetric anesthesiologist Dr. Mark Rosen.

Based on the evidence, discussions of fetal pain for abortions performed before the end of the second trimester should not be mandatory, the researchers said.

The authors include the administrator of a UCSF abortion clinic, but the researchers dispute the claim that the report is biased.

Dr. Catherine DeAngelis, JAMA’s editor-in-chief, said the decision to publish the review was not politically motivated.

“Oh, please,” Dr. DeAngelis said. “If I had a political agenda, I wouldn’t pick fetal pain.”

JAMA does not publish “politically motivated science. We publish data based, evidence-based science,” Dr. DeAngelis said.

The measure pending in Congress would affect about 18,000 American abortions a year performed in the fifth month of pregnancy or later, the legislative director of the National Right to Life Committee, Douglas Johnson, said. He said the review is slanted.

But Dr. Rosen said the researchers “tried to review the literature in an unbiased fashion. This was a multidisciplinary effort by experts on anesthesia, neuroanatomy, obstetrics, and neonatal development.”