Researchers have developed an artificial womb that can keep a very premature sheep foetus alive for four weeks.
The scientists behind the research say that it could lead to similar devices for human foetuses, although that is a long way in the future.
The research is published in Nature Communications and was performed by scientists at the Children's Hospital of Philadelphia in the US.
The scientists mimicked a womb environment by creating a closed plastic bag that brought oxygen and nutrients in via an artificial umbilical cord.
The five lambs, all at the equivalent stage of development of a 23-week human foetus, showed normal development, including lung and brain growth.
Colin Duncan, a professor of reproductive medicine at the University of Edinburgh, told the Science Media Centre that it was a "really attractive concept" and a "very important step forward".
But he warned that there were still "huge challenges" ahead.
"This will require is a lot of additional preclinical research and development and this treatment will not enter the clinic anytime soon," he said. He compared it to the use of steroid injections for women at risk of premature delivery, another development that was first discovered using sheep models.
"It has improved the survival of premature babies worldwide, and made a huge impact on obstetric and neonatal practice," he said, but added that it "took well over 20 years to get into clinical practice".
The researchers say in their paper that clinical applications "will require further scientific and safety validation, and evolution and refinement of the device itself".