Jason Henry for The New York Times
Dr. Mark Rosen, who pioneered anesthesia in fetal surgery, was a co-author of a study concluding that fetal pain was unlikely before the third trimester, which begins at about 27 weeks.
It is a new frontier of the anti-abortion movement: laws banning abortion at 20 weeks after conception, contending that fetuses can feel pain then.
Since 2010, a dozen states have enacted them, most recently Texas. Nationally, a bill passed the Republican-dominated House of Representatives in June.
The science of fetal pain is highly complex. Most scientists who have expressed views on the issue have said they believe that if fetuses can feel pain, the neurological wiring is not in place until later, after the time when nearly all abortions occur.
Several scientists have done research that abortion opponents say shows that fetuses can feel pain at 20 weeks after conception. One of those scientists said he believed fetuses could likely feel pain then, but he added that he believed the few abortions performed then could be done in ways to avoid pain. He and two other scientists said they did not think their work or current evidence provided scientific support for fetal-pain laws.
Some scientists' views have evolved as more research has been done. Dr. Nicholas Fisk, a senior maternal-fetal medicine specialist at Royal Brisbane and Women's Hospital in Australia, said he once considered early fetal pain "a major possibility" after finding that fetuses receiving blood transfusions produced increased stress hormones and blood flow to the brain, and that painkillers lowered those levels.
But Dr. Fisk, a former president of the International Fetal Medicine and Surgery Society, said neurological research has convinced him that pain "is not possible at all" before 24 weeks.
Supporters of fetal-pain laws also say that surgeons' use of anesthesia and painkillers when operating on fetuses in the womb proves fetuses feel pain.
"If the child who is waiting for surgery can feel pain, the child who is waiting for abortion can also feel pain," said Mary Spaulding Balch, the National Right to Life Committee's state policy director, who pioneered fetal-pain laws. She does not advocate performing abortions with anesthesia or painkillers "to have a painless death," but rather wants those abortions prevented because a "member of the human family has reached a point where they are capable of feeling pain."
But fetal surgeons say that the drugs are given for reasons other than fetal pain. Anesthesia is given to the mother, reaching the fetus, said Dr. Scott Adzick, a fetal surgeon at Children's Hospital of Philadelphia. Fetuses receive muscle relaxants and narcotics. The drugs are given to prevent maternal pain, immobilize the fetus, relax the uterus, and block harmful effects of a fetus's hormonal stress response, he said. Asked if fetuses would feel pain without these drugs, Dr. Adzick said he did not know.
Dr. Mark Rosen, who pioneered anesthesia in fetal surgery, said if fetuses could experience surgery-related pain, it would be postoperatively, because during surgery anesthesia from the mother is so strong. Painkillers given to fetuses would prevent postsurgical pain if it exists, but the known scientific reason for them is "it promotes healing," he said. "The fetus is going to go on and develop and live."
Fetal-pain laws usually prohibit abortions at 20 weeks after conception, about 22 weeks gestational age as measured by scientists — from the last menstrual period, since conception cannot be pinpointed. By far most abortions occur before 24 weeks because the Supreme Court has said they may be performed until fetuses can be viable outside the womb, which doctors say is around 24 weeks.
So the laws banning abortions about two weeks before the 24-week threshold affect very few; 98.5 percent of abortions occur before 21 weeks, according to the Guttmacher Institute, a research group that supports abortion rights. Later cases often involve just-discovered birth defects.
Still, both sides consider the laws, which have been challenged and blocked in Arizona, Georgia and Idaho, broader efforts to narrow abortion options and build opposition.
Scientists with varying views of pain development said they did not consider their work applicable to fetal-pain laws or the highly charged abortion debate.
For example, one of the main scientists whose work is cited as support for the laws is Dr. Kanwaljeet Anand, a professor of pediatrics, anesthesiology and neurobiology at the University of Tennessee's Health Science Center. Dr. Anand said he considered fetal pain likely at 18 to 24 weeks. But he added that the "issue of fetal pain does not have much relevance for abortion, since most abortions are performed before the fetus is capable of experiencing pain" and that for the "very few" abortions that occur after that time, techniques could be used that he believed would prevent pain.
Because so much about pain is undetermined, most scientific discussions eventually run into theoretical territory. Much research involves children or adults, making implications for fetuses unclear. Scientists say characterizing responses as pain is difficult; some fetal reactions are simply reflexes or stress hormones. Some researchers say pain-sensing ability depends on consciousness, which may itself emerge gradually.
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