"It's excruciating," he said. "Not only is sensitivity to pain higher in the fetus, it doesn't know when the pain is going to end."
Derbyshire disagrees. Most studies, in his view, demonstrate that the nerve circuitry for pain isn't completely developed until 26 weeks in the womb. That's about when the third trimester begins. But even then, he said, a fetus can't experience true pain without consciousness and context.
"Pain isn't something that just falls out of nervous tissues," he said. "This is a psychological experience. There needs to be coherence."
In a letter to the Nebraska legislature, Mark Rosen -- director of Obstetrical Anesthesia at the University of California, San Francisco -- argued that the evidence is too weak to support the new law. His own review of evidence, published in the Journal of the American Medical Association in 2005, suggested that fetuses are not capable of feeling pain until at least 29 weeks into pregnancy.
"There are no new data to support the conclusions made by the Nebraska legislature," he said. "This is a controversial issue because the available data do not allow absolute conclusions. Unfortunately, there are considerable emotional and political issues at work here."