There's hope for extremely premature babies as scientists develop an artificial womb that already appears to have helped premature lambs develop.

Researchers are creating an artificial womb to improve care for extremely premature babies as remarkable animal testing suggests the first-of-its-kind watery incubation so closely mimics the uterus it just might work.

Premature infants weighing as little as a few hundred grams are hooked to ventilators and other machines inside incubators.

Children's Hospital of Philadelphia is aiming for a gentler solution, to give the tiniest preemies a few more weeks cocooned in a womb-like environment - treating them more like foetuses than newborns in hopes of giving them a better chance of healthy survival.

The researchers created a fluid-filled transparent container to simulate how foetuses float in amniotic fluid inside the uterus, and attached it to a mechanical placenta that keeps blood oxygenated.

The team reported on Tuesday that in early-stage animal testing, extremely premature lambs grew, apparently normally, inside the system for three to four weeks.

"We start with a tiny foetus that is pretty inert and spends most of its time sleeping. Over four weeks we see that foetus open its eyes, grow wool, breathe, swim," said Dr Emily Partridge, a CHOP research fellow and first author of the study published in Nature Communications .

"It's hard to describe actually how uniquely awe-inspiring it is to see," she added in an interview.

Human testing still is three to five years away, although the team already is in discussions with the US Food and Drug Administration.

"We're trying to extend normal gestation," said Dr Alan Flake, a foetal surgeon at CHOP who is leading the project and considers it a temporary bridge between the mother's womb and the outside world.

Increasingly hospitals attempt to save the most critically premature infants, those born before 26 weeks gestation and even those right at the limits of viability between 22 to 23 weeks.

Extreme prematurity is a leading cause of infant mortality, and those who do survive frequently have serious disabilities such as cerebral palsy.

Flake's goal is for the womb-like system to support the very youngest preemies just for a few weeks, until their organs are mature enough to better handle regular hospital care like older preemies who have less risk of death or disability.

The device is simpler than previous attempts at creating an artificial womb, which haven't yet panned out.

The researchers tested five lambs whose biological age was equivalent to 23-week human preemies, and three more a bit older.

All appeared to grow normally, with blood pressure and other key health measures stable and few complications during the weeks they were inside the womb-like device.

The study didn't address long-term development and most of the lambs were euthanised for further study that found normal organ development for their gestational age.

One was bottle-weaned and is now more than a year old, apparently healthy and living on a farm in Pennsylvania.