Jessica Green was getting impatient. She was 19 weeks pregnant and waiting for her ultrasound images at Whitehorse General Hospital, but it was taking forever. She’d never had to wait this long before. Her fiancé, Kris Schneider, had already headed back to work for the day, and Green wanted to do the same. She told the receptionist that she would pick up the images later and headed out. It was late October in Whitehorse, the capital city of Canada’s northern Yukon Territory, and winter was beginning to set in.

The ultrasound technician caught up to her in the parking lot. Green couldn’t leave, the tech said. She needed to be admitted, right away. Green remembers responding with some sort of instinctive, mulish refusal: “I can’t.”

But she knew her pregnancy was considered high-risk: She was 37, she’d conceived via IVF, and she was carrying twins. She followed the tech inside and headed up to the maternity ward, where she learned that her cervix was shortening precipitously, a precursor to labor—it was already down to 1.1 centimeters, less than half of what it should have been. A baby’s lungs and guts take a long time to fully develop in the womb, and her tiny babies still lacked the abilities to breathe or digest food on their own. But the barrier between them and the outside world was fading away.

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April 2018. Subscribe to WIRED. Nik Mirus

Within a few days, a doctor performed an emergency cervical cerclage—effectively, he sewed her cervix shut—to protect the twins. That procedure came with serious risks: Both twins might die. But doing nothing might also mean losing them, so Green and Schneider had opted for action. After the surgery, Green gritted her teeth through a week of strict bed rest at home, but then pain and heavy bleeding chased her back to the hospital, where she was admitted and given morphine, fentanyl, and laughing gas while the staff waited to see if her labor would hold off. When she began to dilate again, the doctors removed the cerclage sutures before they could tear through her cervix. She and Schneider now lived in her hospital room. Contractions, irregular but powerful, came and went for days.

All hope of the twins reaching full term was gone. The couple simply hoped to reach what neonatologists call the threshold of viability: the point at which medical science has the ability to keep a premature baby alive outside the womb.

A full-term human baby can seem helpless at birth, but in comparison to a preemie that baby has an impressive toolkit of skills. Aside from their underdeveloped lungs and guts, babies born too early don’t yet have the reflexes or muscular control to suck and swallow simultaneously. They are prone to cranial hemorrhage, and sometimes a heart duct remains open. Their skin is thin and fragile; the veins glow eerily. They are sensitive to sound, to light, to touch. Their eyelids may still be fused shut, and the tiniest preemies may not yet even have the ability to close a fist around your finger—that essential early act, the moment when they take possession of you.

Over several decades, doctors and nurses have become better at grappling with all of these obstacles. The threshold still varies widely depending on a baby’s circumstances and on the care available immediately at birth. But advances in drugs, technology, and methods of care have pushed that line earlier and earlier, and today there are preemies growing up, healthy and whole, whose survival would have been unimaginable a generation ago. These days, the line between birth and death generally lies somewhere between 22 and 25 weeks’ gestation. Green and Schneider could only pray that they would get there.