The Texas legislature is again debating a bill that would place significant restrictions on the availability of abortions. In Roe v. Wade (1973), the U.S. Supreme Court found that access to abortion is a "fundamental right" that state governments could only limit based on "compelling state interests." The Court found no such compelling interests during the first trimester of pregnancy; during the second trimester, it continued, states may enact regulations to solely protect the health of mother. Only at the point of fetal viability—about 24 weeks—did it declare that the state has an interest in protecting the life of the fetus.

In a gesture toward constitutional plausibility, the Texas bill declares that "substantial medical evidence recognizes that an unborn child is capable of experiencing pain by not later than 20 weeks after fertilization." Because of this, the legislation further claims, there is "a compelling state interest in protecting the lives of unborn children from the stage at which substantial medical evidence indicates that these children are capable of feeling pain." The bill adds that this compelling interest is separate from the compelling interest to protect fetuses at the stage of viability.

Consequently, the Preborn Pain Act prohibits an abortion if a physician determines that the "probable post-fertilization age of the unborn child is 20 or more weeks." It's worth noting that in June the U.S. House of Representatives passed the similar Pain-Capable Unborn Child Protection Act by a largely party-line vote of 228 to 196. The bill has no chance of getting out of the U.S. Senate.

The latest report from the Centers for Disease Control and Prevention says that in 2009, 64 percent of abortions were performed before 8 weeks' gestation, and 91.7 percent before 13 weeks. Seven percent were performed between 14 and 20 weeks, and just 1.3 percent after 21 weeks. Out of 784,000 abortions, just over 10,000 were performed after 21 weeks' gestation. There are not good data on why some women wait until after 21 weeks before choosing abortion, but two likely reasons come to mind: a prenatal discovery of significant fetal abnormalities, and embarrassed adolescent denial.

One more noteworthy piece of data: The youngest surviving premature baby so far is Amillia Taylor, who was born weighing less than 10 ounces and measuring less than 10 inches long via an emergency Caesarean surgery at 21 weeks and 6 days. A 2009 Swedish study reported that 93 percent of infants born at 22 weeks died. At 23 weeks the mortality rate fell to 66 percent, and at 24 weeks it was 40 percent.

For purposes of this discussion, let's set aside any philosophical questions about the conclusions we should draw from the possibility that a fetus feels pain, and simply assume fetal pain may be morally relevant. So do fetuses feel pain at 20 weeks or not?

The preliminary question is: What is pain? The International Association for the Study of Pain defines it as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage." The definition also notes that "pain is always subjective" and that "it is unquestionably a sensation in a part or parts of the body, but it is also always unpleasant and therefore also an emotional experience." In addition, "activity induced in the nociceptors and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state." Nociceptors are nerves that detect damage from mechanical, chemical, thermal, and other stresses. The crucial thing to notice here is the association's insistence that pain is subjective; it is a conscious experience.

So are fetuses capable of having conscious experiences? In support of the Pain-Capable Unborn Child Protection Act, the House Judiciary Committee cited various studies for the assertion that 20-week old fetuses feel pain. One of the more eloquent of the experts who testified on behalf of the bill was Dr. Maureen Condic, an associate professor of neurobiology and adjunct professor of pediatrics at the University of Utah School of Medicine. Condic agrees that the psychological aspects of pain are important but counters that "we can measure certain physical, neurological, and endocrine responses to painful stimuli." She then outlines the developmental process of a fetal nervous system including the "most primitive response to pain, the spinal reflex," which is in place by 8 weeks. By 18 weeks, nerve connections between the spinal cord and thalamus in the developing brain are completed. The thalamus is generally considered to be the part of the brain that relays sensory data to the cortex, which is the outer layer of the brain generally associated with higher mental functions such as thought and action.

Condic does acknowledge that the "long-range connections within the cortex that some believe to be required for consciousness do not arise until much later, around 22-24 weeks." But she believes that the fetal neural structures needed to detect noxious stimuli are in place by 8 to 10 weeks of development. She further asserts: "There is universal agreement that pain is detected by the fetus in the first trimester. The debate concerns how pain is experienced, i.e., whether a fetus has the same pain experience as a newborn or an adult would have." As evidence that it is possible to feel pain without a cortex Condic cites the fact that children born without a cortex and animals whose cortices have been removed will withdraw from pinches, burns, and so forth. As further evidence for fetal pain, Condic cites studies showing that various medical treatments applied to fetuses in the womb boost their stress hormone levels.

On the basis of this evidence, Condic contends, "Direct experimental evidence from adult humans contradicts that the assertion…that mature pain perception requires cortical circuitry." Her conclusion actually rather begs the question of whether perception equals experience.

Most researchers agree with Condic on the developmental course of the fetal brain, but they come to a very different conclusion with regard to fetal pain. In her testimony, Condic criticized three comprehensive analyses of fetal neurological development and its implications for fetal pain—one by the American Congress of Obstetricians and Gynecologists, one by the Journal of the American Medical Association (JAMA), and one by the United Kingdom's Royal College of Obstetricians and Gynaecologists (RCOG).

The RCOG's report, Fetal Awareness: A Review of Research and Recommendations for Practice was issued in March 2010. "In reviewing the neuroanatomical and physiological evidence in the fetus," it found, "it was apparent that connections from the periphery [of the fetal body] to the cortex are not intact before 24 weeks of gestation and, as most neuroscientists believe that the cortex is necessary for pain perception, it can be concluded that the fetus cannot experience pain in any sense prior to this gestation." In other words, while fetuses can react to pain, at the 24-week stage of brain development there is no subject present that is capable of experiencing pain.

The RCOG report also found that even after 24 weeks of development, fetuses abide "in a continuous sleep-like unconsciousness or sedation" that "can suppress higher cortical activation in the presence of intrusive external stimuli." Since fetuses cannot experience pain before 24 weeks, the RCOG recommends against administering pain-relieving drugs when treating fetuses in the womb except in cases when it's necessary to immobilize them.

The 2005 JAMA review came to a similar conclusion: "Evidence regarding the capacity for fetal pain is limited but indicates that fetal perception of pain is unlikely before the third trimester. Little or no evidence addresses the effectiveness of direct fetal anesthetic or analgesic techniques." The JAMA article focused on the subjective nature of pain, noting that the "psychological nature of pain also distinguishes it from nociception [the detection of noxious stimuli], which involves physical activation of nociceptive pathways without the subjective emotional experience of pain." In a June 20, 2013, statement, the American Congress of Obstetricians and Gynecologists cited the JAMA review's conclusion that "fetal perception of pain is unlikely before the third trimester" and added, "Although ultrasound monitoring can show intrauterine fetal movement, no studies since 2005 demonstrate fetal recognition of pain."

The political controversy over fetal pain isn't about the scientific debate as much as it's about how people feel about the morality of abortion. It's a pretty safe bet that people who worry about fetal pain are actually morally opposed to nearly all abortions. They are using whatever scintillas of scientific evidence they can scrounge up to try to justify "compelling state interests" with the aim of prohibiting as many abortions as they can. On the other hand, people who oppose restrictions on a woman's right to control her own body have every reason to argue that the scientific evidence indicating a lack of fetal pain undercuts claims for "compelling state interests" to limit abortions.

Scientific data on whether or not fetuses feel pain simply cannot tell you if abortion is moral or not. In the June 2013 statement, the American Congress of Obstetricians and Gynecologists declared, "Personal decision-making by women and their doctors should not be replaced by political ideology." With abortion, as on so many other contemporary policy issues implicating scientific data, such a replacement is already well underway.

See Reason TV's debate among libertarians over the morality of abortion below.

Disclosure: I am pro-choice and have from time to time made contributions to various pro-choice organizations.