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Scientists have estimated that pain becomes possible anywhere from 18 to 29 weeks into gestation, maybe later. Getty Images

THE GIST

- A new Nebraska law bans abortions after 20 weeks, based on the idea that pain begins then.

- Scientists continue to debate when and how fetuses begin to feel pain.

- Findings could alter the way doctor perform surgery on fetuses, infants and pregnant mothers.

Nebraska recently became the first state to ban abortions after 20 weeks of pregnancy based purely on the idea that fetuses start to feel pain around that time. The law, which adds yet more fuel to an already fiery abortion debate, opened a brimming can of scientific worms.

It turns out that no one really knows what it feels like to be a fetus. Through different interpretations of accumulating evidence, various scientists have estimated that pain becomes possible anywhere from 18 to 29 weeks into gestation, maybe later.

While the debate is essential for doctors who perform fetal surgery, scientists on both sides say their work should not play a role in political matters. Too often, people twist what we know into what they want to believe.

"For decades, the question of fetal pain has been very much overlaid by the implications that it has for abortion," said Kanwaljeet Anand, director of the Pain Neurobiology Laboratory at the University of Tennessee Health Science Center in Memphis.

Anand did much of the formative work suggesting that fetuses truly feel pain and that they feel it remarkably early. "This has nothing to do with abortion," he said.

Stuart Derbyshire, a psychologist and fetal pain expert at the University of Birmingham in the United Kingdom, expressed a stronger view.

"Basing laws on this is really unreasonable," said Derbyshire, who, unlike Anand, thinks pain takes much longer to develop. "Abortion is not a scientific question. It is a moral and political question. To try and make science answer a moral question like that is just wrong. It's cowardice on the part of lawmakers."

A full-term pregnancy lasts about 40 weeks. Premature babies can survive if born after about 22 to 24 weeks, though chances of survival are better after 26 weeks.

One of the first clues that fetuses might feel pain came in the early 1990s, when researchers in England stuck needles into second-trimester fetuses and observed the release of pain-related hormones and nerve-signaling molecules. Before that, doctors thought the fetal nervous system was too undeveloped to feel pain. Even newborns endured surgeries without anesthesia.

"If you or I had that experience," Derbyshire said, "we would wake up and complain loudly."

Scientists have estimated that pain becomes possible anywhere from 18 to 29 weeks into gestation, maybe later.Getty Images

Some of Anand's earliest research showed that newborns were far more likely to survive operations when given anesthetics. That made him wonder what happens before birth.

Since then, he said, studies have shown that the fetal brain and body are coordinated enough to experience pain by between about 18 and 20 weeks. When a fetus of that age gets a blood transfusion, for example, changes in heart rate and blood pressure accompany shifts in circulation and spikes in stress hormones. A morphine-like drug calms all of those responses down.

"The die-hards will say these are all reflexes," Anand said. But new evidence, he argued, suggests that the very young brain is developed enough in the right places to take in those sensations and translate them into pain.

"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."

Putting abortion aside, if possible, fetal pain research has the potential to transform rare, but life-saving surgeries in the tiniest patients. So far, scientists know very little about the long-term effects of drugs on fetuses, preemies and newborns.