Marking what could be a vital development for treating premature babies, researchers announced today that eight fetal lambs survived and grew inside an artificial womb for four weeks, the longest an animal has done so.

The team reports that the lambs’ lungs and other organs developed as though they were in their mother’s womb, an important improvement over the incubators and ventilators now used to keep preemies alive. A few of the lambs have since grown to adulthood, and one is now more than a year old. (Also read about the first year of a baby’s brain development in National Geographic magazine.)

“They appear to have normal development in all respects,” says study leader Alan Flake at Children’s Hospital of