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.”