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 anti-abortion advocates 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.

Stress hormones do not necessarily indicate pain, and now 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 leading 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’ hormonal stress response, he said. Asked if fetuses would feel pain without these drugs, 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. The overwhelming majority of abortions occur before 24 weeks, when the Supreme Court has said fetuses might be viable outside the womb.

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. Anand said he considered fetal pain likely at between 18 and 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.

"There’s far more we don’t know than we do know," said Rosen, who in 2005 co-authored an influential analysis of many studies, which appeared in the Journal of the American Medical Association. It concluded that fetal pain was unlikely before the third trimester, which begins at about 27 weeks. The report said pain sensation required neural connections into the cortex. The cortex begins emerging around the 23rd week, is not functionally developed until the 26th or later, and continues developing after birth, Rosen said.

In 2010, another large review, by a diverse panel appointed by Britain’s Royal College of Obstetricians and Gynaecologists, said "most neuroscientists believe that the cortex is necessary for pain perception" so "the fetus cannot experience pain in any sense prior" to 24 weeks.

In 2012, the American Congress of Obstetricians and Gynecologists endorsed this reasoning, saying studies cited by fetal-pain law supporters were not persuasive "when weighed together with other available information."

Fetal-pain law advocates argue the cortex is unnecessary to feel pain, and the thalamus, developed around 20 weeks, is sufficient. Balch, of the National Right to Life Committee, cites as a strong impetus for the laws a 2007 article in the journal Behavioral and Brain Sciences by Dr. Bjorn Merker, a Swedish neuroscientist. It included observations of five children with hydranencephaly, in which most or all of the cortex is missing, replaced with fluid. Merker, who accompanied the children to Disney World, wrote that they appeared to smile and cry, and theorized that "the tacit consensus concerning the cerebral cortex as the ‘organ of consciousness’ may "be seriously in error."

Balch said this research, involving children ages 1 to 5, "is persuasive evidence that the unborn child feels pain at 20 weeks."

Merker, however, said by email that his research had only "marginal bearing" on fetal pain and "did not deal with pain specifically." He added, "I was not aware that I had been cited in connection with the abortion issue," and would say nothing further about it "because of the way that issue has been infected by politics."

Anand believes the cortex is not necessary for fetal pain, saying some adults have continued feeling pain after cortex tissue removal, and others have had pain eliminated when just thalamus nerves were removed. "It seems that the cortex is not that important even in the adult," he said. "Why do you think it is so important in the fetus?"

But Rosen, a professor emeritus of anesthesiology, obstetrics and gynecology at the University of California, San Francisco, said pain involves "complex feedback loops between different structures," so pain receptors must extend through the spinal cord and thalamus into the cortex. "You can make a telephone call, but not till wires that connect our phones exist," he said. "You can say the wire now exists, but nobody’s turned the service on."

On both sides, viewpoints continue evolving. Stuart Derbyshire, a psychologist at the National University of Singapore, is a leading voice against the likelihood of fetal pain. Appointed to the British panel, he strongly supports its main conclusion, but increasingly doubts another suggestion in the report, that the uterine environment may keep fetuses in "continuous sleep-like unconsciousness or sedation."

That, he said, was "a really appealing argument, but unfortunately I don’t think it quite works."

Anand’s fetal-pain theories grew from important research he did with premature infants showing that the then-common practice of not anesthetizing infants undergoing surgery caused hormonal stress responses and impeded recoveries. This did not prove the infants felt pain, but it largely contributed to making anesthesia standard because lowering stress responses helped babies survive.

Most premature infants are older than fetuses the laws refer to, with more-developed brains. But Anand said he believes fetuses may feel pain through different pathways, possibly the subplate, a way station for budding neurons, which later folds into the cortex. Rosen said the subplate theory was "unsubstantiated conjecture," likely too simplistic for pain sensation.

In 2004, Anand testified about fetal pain in trials on laws banning late-term procedures, which opponents call partial-birth abortions. In 2005, he testified before a congressional committee considering fetal-pain legislation.

Recently, he said he had intended to highlight science, not support anti-abortion views. Since 2005, "I’ve been asked to testify many, many times, and I’ve turned it down," he said. "I feel it’s just gotten completely out of hand."

He said women and their doctors should make abortion decisions case by case. "In the very few abortions where fetal pain could possibly occur," he said, "consider what can be done to avoid inflicting a lot of pain on the fetus."

He said a common method used after 20 weeks — injecting amniotic fluid or fetuses with heart-stopping medication before removing the fetus — "would be fine, really, from a point of view of fetal pain," a "compassionate way to do it." Anti-abortion advocates disagree, equating the injections with heart attacks.

Anand said that if injections could not be done, he would recommend "some sort of fetal anesthesia."

Dr. Daniel Grossman, vice president for research at Ibis Reproductive Health, who performs abortions, said fetal anesthesia is rare, carrying risks to mothers, but if patients who understood the risks asked, he would provide it.

Dr. Anne Davis, consulting medical director for Physicians for Reproductive Health, who performs abortions until 24 weeks, said publicity about the laws has prompted patients to ask about fetal pain. She tells them "there won’t be" any because "the best evidence is the fetuses are not feeling pain until pretty much into the third trimester."

Some scientists say if fetuses feel pain, childbirth would seem to be particularly painful. Yet fetal-pain law supporters do not advocate fetal anesthesia or painkillers then. Balch said she believes "there is something that is produced that prevents pain" for babies being born.

Scientists say that’s not so. "There are ways in which the pain of being born may help the fetus by producing activation in the pathways of the brain," Anand said.