A human fetus is unlikely to feel pain before the third trimester, when consciousness begins to form, researchers said on Tuesday in a report that could fuel debate over proposed U.S. abortion legislation.

Even if a fetus feels pain, doctors may not be able to anesthetize it without endangering the mother’s health, including during an abortion, the researchers wrote in the Journal of the American Medical Association.

Legislation under consideration by the U.S. Congress and some U.S. states would require doctors to inform women seeking abortions after the 22nd week of gestation that their fetus feels pain and offer to anesthetize the fetus.

Supporters of the legislation say that when a fetus displays a withdrawal reflex or hormonal stress response, that is evidence of fetal pain. But the researchers at the University of California, San Francisco, questioned that view, saying the responses may be automatic and not signs of discomfort.

Drawing on findings from thousands of medical-journal articles on the subject of fetal pain and related topics, the report’s author, Susan Lee, wrote that “pain is a subjective sensory and emotional experience that requires the presence of consciousness.”

Consciousness is created by brain connections between the thalamus and the cerebral cortex, and those do not begin to develop before the 23rd week and possibly not before the 30th week of gestation. The human gestation period is 38 weeks from conception.

“Conscious perception of pain does not begin before the third trimester,” Lee wrote. In the United States, only 1.4 percent of abortions are performed at or after 21 weeks gestational age, the report said.

Anesthesia is used in some surgeries where the fetus is operated on inside the womb, but it is not done to address fetal pain, Lee wrote. It is designed to relax the uterus, immobilize the fetus, or ease stress if surgical complications occur.

In addition, the dose needed to anesthetize a fetus might endanger the mother by slowing her breathing excessively, the report said, adding new anesthetizing techniques would have to be developed to address fetal pain, if it exists.

Dr. Wendy Chavkin of Physicians for Reproductive Choice, and Health, commenting on the report, said its conclusions affirmed what other experts have found, and denounced the proposed legislation concerning fetal pain.

“These laws have nothing to do with pain or pain reduction, but are clearly intended to stigmatize abortion, the women who have abortions and the doctors who provide them,” she said.