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The 24-week line was a guess
Why viability became the line
When Roe v. Wade balanced a woman's right to choose against the state's interest in an unborn child, the Supreme Court needed a moment in a pregnancy where the second took over. The best objective medical standard the justices could find was viability: whether the fetus could survive outside the womb.
Charts of fetal development gave them the markers. The brain and heart begin to develop at three weeks, nerve fibers appear at eight, moving and kicking start at 13 to 16, some fetuses respond to familiar sounds like a mother's voice at 23, and air sacs form in the lungs at 24. So 24 weeks looked like the earliest point a fetus might breathe on its own. No one was testing that, because no one was taking fetuses out at 24 weeks to see if they would survive.
How surfactant moved it
Through the 70s and 80s, most hospitals drew the real line at 28 weeks, when the lungs start producing surfactant, a fluid that keeps them from collapsing and sticking together between breaths. In the late 70s, an artificial surfactant arrived, a slippery white substance shot down the ventilator tube at birth. Combined with IV nutrition and very small ventilators, it moved the line back to about where it sits now.
Today the line is generally 24 weeks, though it can be 23 or 25, and in some cases 22, depending on the hospital and the doctor on call. Picture an artificial womb that works. If viability means surviving outside the womb with any technology available, the line could slide all the way back to a fertilized egg.
How did artificial surfactant change survival?
Surfactant lowers surface tension inside the lungs' tiny air sacs, so they stay open instead of collapsing like a wet balloon. Mary Ellen Avery and Jere Mead showed in 1959, in the American Journal of Diseases of Children, that babies dying of respiratory distress syndrome lacked it. Tetsuro Fujiwara and colleagues in Japan later reported the first clinical success with an animal-derived surfactant in The Lancet.
Why did the Court tie the line to viability?
In Roe v. Wade in 1973, Justice Harry Blackmun wrote that the state's interest in potential life becomes compelling at viability, because the fetus then has the capacity for meaningful life outside the mother's womb. The opinion said viability is usually placed at about seven months, or 28 weeks, but may occur earlier, even at 24 weeks.
How do ventilators suit tiny lungs?
Ventilators for premature babies deliver very small puffs of air, measured in a few milliliters per kilogram, because stretching delicate lungs can injure them. Many units now begin with gentler breathing support such as CPAP when they can. William Northway first described bronchopulmonary dysplasia, a chronic lung disease of ventilated premature babies, in a 1967 paper in the New England Journal of Medicine.
Could an artificial womb move the line?
In 2017, researchers at the Children's Hospital of Philadelphia reported in Nature Communications that a fluid-filled bag connected to an artificial placenta kept premature fetal lambs alive for up to four weeks. The lambs were at a stage of lung development comparable to a human fetus at about 23 to 24 weeks. During that time their lungs and brains continued to develop.
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