Correction: The Texas Senate removed a ban on abortion after 20 weeks of pregnancy before approving an abortion bill this week. An editing error wrongly reflecting the Senate’s action has been removed from its online versions.This week the U.S. House of Representatives voted to outlaw abortion at 20 weeks post-fertilization. That’s two weeks earlier than the common measure of fetal viability, the line previously drawn by the Supreme Court. Why 20 weeks? Because, according to Republican lawmakers, that’s the point at which science shows that the fetus is sufficiently developed to experience pain.Actually, the science is much more complicated. The neural circuitry that culminates in pain perception begins to form long before 20 weeks, and most researchers believe the signals conveyed by this circuitry aren’t perceived as pain until the brain develops that perceptive capacity well after 20 weeks. The evidence is opaque and debatable. What’s clear is that 20 weeks is just a politically convenient line drawn, for the time being, along this continuum. If pro-lifers can nudge Congress, the public and the Supreme Court from the viability marker to the pain marker, they’ll keep pushing the line backward until abortion is completely illegal.Rep. Marsha Blackburn, R-Tenn., managed the House debate for pro-lifers. “We know that at eight weeks, babies feel pain,” she told her colleagues.Eight weeks? What happened to 20? The answer is that fetal pain is ambiguous. You can stretch the definition to justify banning abortions much earlier. Maureen Condic, the pro-lifers’ go-to neuroscientist, laid out the timeline at a recent House subcommittee hearing:4 weeks: Basic structures of nervous system established. Earliest neurons in the cortex are born.7 weeks: Synapse formation begins in cortex.8-10 weeks: Spinal circuitry for pain detection is established. The fetus is capable of reacting to painful sensory input. Subcortico-frontal pathways established.12-18 weeks: Spino-thalamic pathways established. The fetus is capable of mature pain perception.22-24 weeks: Long-range cortical projections form.None of these milestones occurs at 20 weeks. The timeline is an invitation to shift the legal limit backward from 20 weeks to 18 to 12 to eight and eventually to four. Once you accept pain as the legal standard, the definition of pain can be broadened from actual pain perception to the spinal capacity for pain perception to the capacity to react to painful stimuli, even if the fetus doesn’t experience them as pain. If you don’t buy these claims about pain, pro-lifers will tell you that early fetuses should nevertheless be legally protected on the grounds that they’ve developed a sense of touch. If you don’t accept the claims about tactile sensation, pro-lifers can cite other milestones. Fetal development moves forward along this continuum, but the pro-life political agenda moves backward. First you draw the line at viability. Then you draw it at pain capacity. Then you draw it at thumb-sucking. Then you draw it at hearing. Then you draw it at kicking. Then you draw it at heartbeats. The lines are fuzzy because the baby’s systems form slowly, allowing you to backdate your interpretation to the earliest function that looks like a heartbeat, kick, or cringe.Ultimately, pro-lifers plan to outlaw abortions at conception. They believe, in the words of Rep. Ann Wagner, R-Mo., that “killing an unborn child is unacceptable at any time.”But to get there, they need you to start small, by letting go of the viability standard and shifting to the pain standard. Then it’s all downhill. William Saletan covers science, technology and politics for Slate.