The growth of a baby lamb within four weeks (File photo)

American scientists have managed to keep premature baby lambs alive for weeks using an artificial womb that resembles a plastic bag.

The bag, developed by a team at the Children's Hospital of Philadelphia, contains a mixture of warm water and salts, similar to amniotic fluid, which supports and protects the babies. The eight lambs were able to breathe and swallow like their naturally conceived counterparts after being removed from the "wombs."

The plastic bags cannot replace real wombs or extend the limits of viability; they were just developed to find a better way to support premature infants, according to the research team.

Experts hope the approach might one day help premature human babies have a better chance of survival. Currently, extremely premature infants, born after around 23 weeks of gestation, are placed in incubators and put on ventilators to help them breathe, but this can damage their lung development.