Scott Gilbert was walking through the halls of Swarthmore when he saw the poster, from a campus religious group: “Philosophers and theologians have argued for centuries about when personhood begins,” it read. “But scientists know when it begins. It begins at fertilization.” What troubled Gilbert, who is a developmental biologist, was the assertion that “scientists know.” “I couldn’t say when personhood begins, but I can say with absolute certainty scientists don’t have a consensus,” he says.

When life begins is, of course, the central disagreement that fuels the controversy over abortion. Attacks on abortion rights are now more veiled and indirect---like secret videos pointing to Planned Parenthood’s fetal tissue donations, or state legislation that makes operating abortion clinics so onerous they have to shut down. But make no mistake, the ultimate question is, when does a fetus become a person---at fertilization, at birth, or somewhere in between?

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Here, modern science offers no clarity. If anything, the past century of scientific advances have only made the answer more complicated. As scientists have peered into wombs with ultrasound and looked directly at sperm entering an egg, they’ve found that all the bright lines they thought existed dissolving.

The Quickening

Before ultrasounds and long before Roe v. Wade, it was obvious when life began. The “quickening,” the first time a woman felt her baby’s kick, was the moment the baby came alive, the moment it got a soul. When Henry VIII’s wife felt her quickening, it was cause for celebratory bonfires across London. In the 19th century, abortion in Britain was legal---until the quickening.

But the importance of the quickening---a concept that had been around since at least Aristotle---is now a relic. Before a mother can feel her baby kick, at around 20 weeks, she can already hear its heartbeat and see the blurry outline of its face with ultrasound. In a 2012 vice presidential debate, Paul Ryan explained his views on abortion by talking about seeing the bean shape of his unborn daughter on an ultrasound. He and his wife nicknamed her “Bean.” Ryan would later sponsor a bill for fetal personhood, which gives full legal rights to a zygote after fertilization.

In a way, science made possible the argument for fetal personhood. It's only tenable because people can peer inside the womb, at one time a black box. Indeed, when American physicians began collecting humans embryos and charting embryonic development in the late 19th and early 20th centuries, they began considering fertilization as the beginning of fetal life. Around the same time, writes historian Sara Dubow in her book Ourselves Unborn: A History of the Fetus in Modern America, some physicians began to argue that abortion should be illegal. (Dubow declined to be interviewed for this story, citing concerns about being misquoted on abortion politics.)

When Is Fertilization?

The next century of developmental biology made things even more complicated. With in vitro fertilization---combining sperm and egg in a lab---scientists could directly observe the process of sperm entering the egg for the first time. It actually takes place over as long as 24 hours; a series of biochemical changes need to happen before the sperm can enter. Inside the body, fertilization can happen hours or even days after insemination, as the sperm travels up the fallopian tube. This journey also induces changes in the membrane of the sperm, called capacitation, that ready it to fertilize eggs. (The discovery of artificial capacitation was key to making in vitro fertilization possible.) As the fertilization researcher Harvey Florman has said, “Fertilization doesn’t take place in a moment of passion. It takes place the next day in the laundromat or the library.”

But even fertilization isn't a clean indicator of anything. The next step is implantation, when the fertilized egg travels down the fallopian tube and attaches to the mother’s uterus. “There’s an incredibly high rate of fertilized eggs that don’t implant,” says Diane Horvath-Cosper, an OB-GYN in Washington, DC. Estimates run from 50 to 80 percent, and even some implanted embryos spontaneously abort. The woman might never know she was pregnant.

Assuming that fertilization and implantation all go perfectly, scientists can reasonably disagree about when personhood begins, says Gilbert. An embryologist might say gastrulation, which is when an embryo can no longer divide to form identical twins. A neuroscientist might say when one can measure brainwaves. As a doctor, Horvath-Cosper says, “I have come to the conclusion that the pregnant woman gets to decide when it’s a person.”

The Changing Threshold of Viability

Roe v. Wade allows abortion up to the point a fetus is viable outside the womb. But that's not much help, either. When Edward Bell, a neonatologist at the University of Iowa Children’s Hospital, first began practicing medicine in the 70s, the line was 26 weeks. “The threshold has decreased by one week for every decade,” Bell says. “In the 90s we were all confident that 24 weeks was going to be absolute limit because of the biology.”

But earlier this year, Bell published a paper in the New England Journal of Medicine showing reasonably good outcomes in preemies born at 22 weeks of gestational age. Two key technologies have pushed that date: the use of steroids, which can speed up fetal development, and surfactants that prevent lungs from collapsing after birth. Still, setting an absolute cutoff for fetal viability is impossible. “It depends on how you define it. Is it some babies survive? Half survive? Or most babies survive?” Bell says. At 22 weeks, many of the babies that survive end up with permanent health problems or disabilities.

Bell is wary of his research being appropriated by the debate over abortion. To doctors and scientists, the question of when life begins isn’t a matter of gathering more evidence. “The science has very little to do with the answer,” says Gilbert. Every iteration and advance in the lab make the question even more the purview of philosophers and theologians. And lawyers.