For a study published Tuesday in the journal Nature Communications, Partridge and other researchers from Philadelphia suspended premature lambs, a close animal model for human fetuses, in a liquid-filled, artificial womb, allowing them to further develop for four weeks—longer than in past similar attempts.

The researchers used eight lamb fetuses that were 105 to 115 days old—a level of development comparable to a 23-week-old human fetus. As they floated, the lambs’ brains and organs developed normally. The pinkish creatures opened their eyes, fattened up, and grew coats of white wool.

The researchers anticipate the animal studies will be completed within two years, and if approved, the wombs can be tested on human preemies within three to five years.

One reason preterm birth is so dangerous is that, for an underweight baby, the first few breaths of air halt the development of the lungs. “Infants that are currently born and supported in a neonatal intensive care unit with gas-based ventilation demonstrate an arrest of lung development,” Partridge says, “which manifests in a long-term, severe restriction of lung function.”

With the artificial womb, the infant would continue “breathing” through the umbilical cord as its floats in amniotic fluid, which would flow into and out of the bag. Using its tiny heart, the fetus would pump its own blood through its umbilical cord and into an oxygenator, where the blood would pick up oxygen and return it to the fetus—much like with a normal placenta. In addition to boosting lung growth, the amniotic fluid would protect the baby from infections and support the development of the intestines.

The babies who are hooked up to this apparatus would need to be delivered by C-section, as 60 percent of extreme preterm babies currently are. During the operation, the fetus would be given a drug that would prevent it from taking gulps of air during its brief brush with the outside world. Within seconds, it would be submerged again in the polyethylene bag, just like it was in the womb.

“I don’t want this to be visualized as fetuses hanging on the wall in bags,” said Alan Flake, a fetal surgeon at Children’s Hospital of Philadelphia and an author of the study, on a call with reporters.

He says it will look similar to a traditional neonatal incubator, potentially equipped with a camera to assuage anxious parents. The doctors could even pipe in the mother’s heartbeat. It’s “less stressful than seeing an infant on an incubator on an exposed bed,” Flake said. “That’s a very distressing environment for parents.”

Many hurdles remain in translating the lamb research to human babies. Artificial uteruses have been tried—and failed—before. A 1996 New York Times Magazine story declared “the artificial womb exists” and described a Tokyo lab in which baby goats floated in a heated, fake amniotic suspension. But, alas, it all ended in technical issues and circulatory failures.