John A. Robertson holds the Vinson and Elkins Chair at the University of Texas School of Law. He has written and lectured widely on law and bioethical issues, has served on or been a consultant to many national bioethics advisory bodies and is currently chairman of the Ethics Committee of the American Society for Reproductive Medicine.

Roe vs. Wade established the right to terminate a pregnancy up until viability, the time when a fetus could survive outside of the uterus, generally believed to occur at 24 weeks. Before viability, a woman may abort for any reason. After viability, she may abort only if necessary to protect her life or health.

A new front has opened in the abortion wars with laws that ban abortion at 20 weeks on the ground that the fetus is then capable of feeling pain. Led by Nebraska in 2011, 10 states have passed such laws, and the Texas Legislature has recently considered doing so.

Although affecting less than 1 percent of the 1.2 million abortions occurring annually in the United States, abortion between 20 and 24 weeks may be hugely important for women whose fetus has a lethal or severe genetic anomaly or who otherwise find that they cannot continue the pregnancy. If this challenge to Roe vs. Wade is upheld, it would be an important victory for anti-abortion groups, spur more states to enact such laws and signal that the Supreme Court is ready to reconsider other aspects of the abortion right.

Will 20-week bans on abortion because of fetal pain be found constitutional? Lower courts, which are bound by Supreme Court precedents, are likely to strike down such laws, as has the federal Ninth Circuit Court of Appeals with Arizona’s 20-week ban. Ultimately the Supreme Court will have to weigh in on the issue. Its decision will turn first on medical and scientific facts about neurological development and the perception of pain. But even if the facts establish pain sensation in the fetus, there is still the question of its legal significance, since 20 weeks is well short of viability. In sorting through these issues, Justice Anthony Kennedy will likely provide the key vote, as he did a few years ago in upholding a federal ban on partial-birth abortion.

First the neurological facts. Without nerve connections established between the cortex and the nerves in the thalamus that transmit pain signals, reactions to noxious stimuli cannot be interpreted as feeling or perceiving pain. Expert committees in the United States and Great Britain have agreed that the fetus is not pain-capable until 26 weeks or later. A meta-study of fetal pain studies published in the Journal of the American Medical Association concluded “that a fetus’ neurological pathways that allow for the ‘conscious perception of pain’ do not function until after 28 weeks gestation.”

As with the climate change debate, however, a few scientists always disagree with the overwhelming majority. The most noteworthy voice against the dominant view of no fetal pain perception until 26 weeks is Professor K.J.S. Anand of the University of Tennessee Health Science Center. He thinks that fetuses may experience pain based on subsensory cortical and thalamic structures that develop before 20 weeks. Anand criticizes the expert reviews on methodological and substantive grounds, including their failure to recognize the role of a subsensory cortex basis for feeling pain. He believes that fetal development of the thalamus occurs much earlier than the sensory cortex, providing the substrate and mechanisms for conscious pain perception well before the third trimester of human gestation.

The uncertainty about adequate cortical and neurological structure is compounded by different interpretations of the meaning of an organism’s avoidance responses to external stimuli or internal hormonal surges. There is no doubt that fetuses poked and prodded by instruments or noxious stimuli react aversively, but so do other organisms, such as amoeba, for which no claims of pain perception would stand. Similarly with hormonal surges in the face of noxious stimuli. Yes, hormones indicating stress surge, but does that mean that the fetus is perceiving or feeling pain? The problem, of course, is that pain is inherently subjective. Without someone telling us that they are experiencing pain, which the fetus cannot do, we must rely on surrogate markers, some of which are reliable and others not.

Judging from the Supreme Court’s most recent engagement with conflicting facts in an abortion case, it will not be surprising if Justice Kennedy, the key fifth vote, will find that states have a rational basis for thinking that fetuses feel pain. He took that position in Gonzalez vs. Carhart with regard to Congress’ claim that partial-birth abortion never served a valid medical need, despite the heavy weight of medical evidence to the contrary. But Justice Kennedy was careful to point out that the ban on partial-birth abortions did not prevent a woman from having an abortion — it only limited the technique that might be used.

A ban on abortion at 20 weeks is a different matter. No abortion after 20 weeks would then be permissible (except to protect the life or health of the woman). For the court to uphold a 20-week ban it would have to change its consistent pronouncements over 40 years that the right to terminate a pregnancy exists until viability. The question would be why conflicting evidence of sentience should lead it to change that view. Viability is based on survivability outside of the uterus. Sentience might support laws that require anesthetics to be administered before the abortion, but sentience in itself would not make the fetus viable if viability is not possible until 24 weeks.

Whether the viability limit on abortion should stand is challenging because while viability has a strong intuitive appeal, the court was not very specific about its reasons for drawing that line. It simply said that viability had “logical and biological” justification. Indeed, it gave a definition of viability — the ability to survive outside the uterus — instead of a syllogism for why survivability should matter. If viable, the fetus had to stay in the womb even though it could by definition survive outside.

In retrospect, there are good reasons for supporting that line in contrast with banning abortion at earlier stages. Earlier in the pregnancy fetal status was too closely linked with widely varied personal moral, religious, and philosophical views to sustain laws that would limit a woman’s choice over her body and place in society. At viability, however, the fetus is then so well-developed that the idea of protecting it is much less morally arbitrary or subjective than it was before. This did not mean that fetuses became constitutional or legal persons. A state, however, might then accord it the protections given persons after birth if it so chose.

Does ability to feel pain provide a marker for objective assessment akin to viability? Given the weight of medical and scientific opinion against fetuses experiencing pain at 20 weeks, the answer is likely to be “no.” But if there were scientific consensus that neurological structures were in place to feel pain, the question then would be whether fetuses that are not developed enough to survive outside the womb nevertheless could be protected so they could develop to that stage.

It is hard to see why this should follow. The solution would appear to be to ensure that that terminations are done painlessly at 20 weeks, not that they not be done at all. The ability to feel pain would still not be enough to say that the fetus is so well-developed that it now can be treated as a born baby, if it could not otherwise survive outside the womb.

A different view would raise the question of whether grounds other than viability should be the limit on abortions. If the fetus is sentient, is that not a sufficient basis for finding it so well-developed that it could be protected in its own right, even if it could not survive outside the uterus? And if sentience suffices, why not presence of a heartbeat, say at 10-12 weeks? Note that the protection sought would be to continue a pregnancy, not prevent the harm of pain that abortion anesthetics would provide in terminating a nonviable fetus.

Viability has proven to be a line now well-entrenched in law and practice, and in my view should be retained. Changing it because of sentience when the evidence is dubious and anesthesia is available should not persuade a court otherwise disposed to a basic abortion right to move that line. If that basic right is rejected, then arguing about whether the outer limit on abortion should be 24 or 20 weeks would lose its significance.