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The Senate rejected a ban on late-term abortions late Monday. It was an unsurprising outcome, but such a ban is a big priority for anti-abortion-rights politicians in Congress.

The bill would have banned all abortions after 20 weeks on the grounds that the fetus would feel pain and suffer during the procedure.

Abortion is legal in the U.S. and protected as a right to privacy. But why would any woman want such a late-term abortion, anyway? And can a fetus really feel pain?

Dr. Daniel Grossman of the University of California San Francisco boiled it down to a series of tweets.

“Research has shown a fetus does not yet have the capacity to experience pain until at least the third trimester, and unlikely until birth,” he said in one tweet addressed to Senate Majority Leader Mitch McConnell, a Kentucky Republican.

.@SenateMajLdr, you noted that “fetal pain” is your main reason for banning abortion at 20 weeks, and I’d like to take the opportunity to share some evidence-based medical information on this with you, as well. https://t.co/PsoxO8nrsj — Dr. Daniel Grossman (@DrDGrossman) January 26, 2018

“This has become such a highly politicized topic that the facts really get lost,” Grossman told NBC News.

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Scientific studies indicate it’s not biologically possible for a fetus to feel pain, he said.

“The nerve fibers that connect the pain receptors to the cerebral cortex, where they would be able to perceive the pain, those fibers aren’t even present until the third trimester of pregnancy – after 26 or 28 weeks,” he said.

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“So the connections just aren’t there. All the best evidence by researchers in this area indicate that if the fetus can feel pain at all while in the womb, it’s really later, in the third trimester,” he added.

“There is some data to suggest that the fetus is kind of in a semi-anesthesized state throughout all of the pregnancy and that all of the perceptions are blunted.”

Women seek abortions after 20 weeks for many reasons, Grossman said, including a late diagnosis of a severe fetal abnormality that means it would not survive after birth, or be in severe pain.

Women may also need an abortion to save their lives. “Sometimes the patient may develop complications of the pregnancy and the treatment is delivery and when that is at 20 or 21 weeks, that means terminating the pregnancy,” Grossman said.

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“I really feel like it’s important for policymakers, legislators, to use the best available scientific evidence when they are making policy related to health. I also think it’s important for them to listen to the patients that are affected by this healthcare and neither of those things were done related to this recent bill.”

Similar to the fight to pass the Partial Birth Abortion Ban Act and the Unborn Victims of Violence Act, it’s only a matter of time until we secure the votes to pass the Pain-Capable Unborn Child Protection Act. — Lindsey Graham (@LindseyGrahamSC) January 29, 2018

And, Grossman asserted, state laws aimed at protecting unborn babies are in fact forcing women to have later and later abortions.

“It’s also important to recognize that women have to jump through more and more hoops to access abortion care here in the U.S. even though it’s still legal. And those obstacles that are put in their way are creating delays before they can access care,” he said.

Restrictive laws in Texas, for example, led to clinic closures that meant longer waits for abortions. “There was a 27 percent increase in second-trimester abortions,” he said.