Utah recently passed a law that requires doctors to give anesthesia to a fetus prior to performing an abortion that occurs at 20 weeks of gestation or later.

The law assumes that a fetus may be able to feel pain at that stage in development; however, doctors groups and other critics of the law argue that a fetus cannot feel pain at 20 weeks gestational age.

Indeed, the American College of Obstetricians and Gynecologists (ACOG) said it considers the case to be closed as to whether a fetus can feel pain at that stage in development. [6 Myths About Miscarriage]

"The science shows that based on gestational age, the fetus is not capable of feeling pain until the third trimester," said Kate Connors, a spokesperson for ACOG. The third trimester begins at about 27 weeks of pregnancy.

To find out more, Live Science dug into the research and spoke with a leading expert on fetal pain. Here's a look at what we found.

The problem with pain

One reason the question of fetal pain is so controversial is because pain is always a subjective experience, said Dr. Anne Davis, an OB/GYN and the consulting medical director for Physicians for Reproductive Health. Davis is an abortion provider.

Unlike with blood pressure or body temperature, for example, there's no definitive way to measure pain, Davis said. People do have ways of communicating how much pain they're feeling; for example, doctors often ask people to rate their pain on a scale of 1 to 10. But the experience of pain is fundamentally subjective, Davis said. In other words, what might be very painful to one person may cause very little pain to someone else.

Still, even though doctors can't objectively measure pain, research has revealed much about how pain is experienced in the body and, more importantly, in the brain.

"Pain occurs in [the] brain," Davis said. When a person is injured — say, you stub your toe, for example — a signal travels from the foot up through the nerves in the leg to the spinal cord, and then from spinal cord up to the brain, Davis said. Once that signal gets into the brain, the information is transmitted through a complex web of neurons to an area of the brain called the cortex, she said.

It's in this sophisticated part of the brain that a person actually perceives the feeling of pain, Davis said.

"We know that there are a lot of steps in between the thing that could cause pain and the actual experience of pain," Davis said. For the system to work — whether in an adult or a fetus — all of the pathways of the nerves need to be connected and functioning, she said.

Fetal development

"What we can say about the fetal nervous system is that based on the best science we have" on the neurons that carry pain signals is that the "system isn't developed until the third trimester of pregnancy," Davis told Live Science.

Scientists' knowledge of the fetal nervous system was summed up in a 2005 review in the journal JAMA. The authors of that review outlined in detail the evidence on how this system develops, based on a number of previous studies on the anatomy of the fetus at various stages of development.

Davis, who was not involved with that review, noted that though it was published in 2005, the research is still valid, because the scientific community's understanding of fetal development is "pretty much stable." Indeed, since the publication of the review, "no research has contradicted its findings," said a recent statement from ACOG.

In the review, the researchers highlighted several key points in fetal development that are required in order for a fetus to perceive pain. One is that the receptors in the skin that sense an injury must be developed. Research has shown that this happens between 7.5 and 15 weeks of pregnancy, depending on the location of the receptors on the body, according to the review. For example, receptors in the skin around the mouth develop at around 7.5 weeks, whereas receptors in the skin on the abdomen develop at around 15 weeks, according to the review.

Second, the neurons in the spinal cord that transmit that signal up to the brain must be developed. Researchers who looked at fetal tissues reported that this happens at around 19 weeks, the review said.

Third, the neurons that extend from the spinal cord into the brain need to reach all the way to the area of the brain where pain is perceived. This does not occur until between 23 and 24 weeks, according to the review.

Moreover, the nerves' existence isn't enough to produce the experience of pain, the authors wrote in their review. Rather, "These anatomical structures must also be functional," the authors wrote. It's not until around 30 weeks that there is evidence of brain activity that suggests the fetus is "awake."

Davis noted that while these time frames aren't exact — some fetuses may develop a little earlier, and some fetuses may develop a little later — "there isn't any science to suggest that those pathways [for pain] are complete around the 20th week" of pregnancy.

"It's a complicated development process, and it goes in stages," Davis said.

According to a statement from ACOG, a fetus's brain and nervous system "do not have the capacity to process, recognize or feel pain during the second trimester."

Indeed, it's important to remember that early on in pregnancy, the fetus isn't just a very small version of what it looks like later in pregnancy, Davis said. Rather, things are changing and organs are forming, she said. There are number of fetal conditions that can't be diagnosed until later in pregnancy, because the development simply hasn't happened yet, she said.

Reflexes and stress responses

One argument that is sometimes used to suggest a fetus can experience pain before the third trimester is that a fetus can have a withdrawal reflex, or the ability to move away from something when touched.

But performing a reflex action and perceiving pain are two different things, Davis said. Consider, for example, when a doctor tests your reflexes by hitting your knee with a rubber hammer. Your foot will kick out, regardless of whether you experience pain or not.

"Many reflexes occur at the level of the spinal cord," and don't involve the brain at all, Davis said. But the brain is essential for perceiving pain, she said. [5 Painful Facts you Need to Know]

Another argument is that a fetus in the second trimester can display certain stress responses, such as increased levels of stress hormones, including cortisol and endorphins. However, the authors of the JAMA review noted that these hormones aren't specific to pain (for example, other stressful conditions may affect their levels). In addition, the hormones are not regulated by the part of the brain associated with consciousness, the authors wrote.

Doctors react

Utah's law requiring anesthetizing a fetus prior to an abortion also brings up important technical questions: How should doctors perform such a procedure? Is there an added risk to the woman? For example, although it's been shown that painkilling drugs cross the placenta and reach the fetal bloodstream, doctors don't know how much of the drug they would have to give the woman in order to achieve the desired level in the fetus, and if this amount is safe for the woman, the JAMA authors wrote.

There's no protocol for how to do this, Davis said, and experts in the field of maternal medicine aren't sure how to follow this law. Doctors are able to immobilize a fetus to perform certain in-utero surgeries, but this is different than blocking pain in the fetus, according to the JAMA review.

Dr. Leah Torres, an OB/GYN in Salt Lake City, also said that it's not medically possible for doctors to follow this law.

"There is no medical practice that involves administering [pain relief] to a fetus," she told The Salt Lake City Tribune earlier this month.

Yet another issue is that pain is a part of many medical procedures. And so, fundamentally, the law begs another important question: Why does the potential existence of pain mean that a procedure should be avoided? Davis said.

Follow Sara G. Miller on Twitter @saragmiller. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.