The survival rate of babies born before their mothers are 27 weeks pregnant has increased, research suggests.
However, the proportion of premature babies who experience serious health problems has remained "largely unchanged", according to two studies.
The papers, published on bmj.com, suggest that while the pre-term survival rate continues to rise, the number of long-term disabilities, such as learning difficulties and cerebral palsy, will rise in parallel.
The large studies, led by teams at University College London (UCL) and Queen Mary, University of London, compared babies born between 22 and 26 weeks in 2006 with those born between 22 and 25 weeks in 1995.
The first examined immediate survival rates and health. Researchers found that overall survival rates increased by 13% during this period.
However, they found that there was no significant increase in survival rates for babies born under 24 weeks - the current legal timeframe for abortions.
The number of babies which experience major health problems remained unchanged.
The second study examined the health of the premature babies when they were three years old.
Researchers found that in 2006, the proportion of survivors born between 22-25 weeks with severe disability was roughly the same as it was in 1995.
Professor Neil Marlow, a Medical Research Council-funded researcher from the UCL Institute for Women's Health and an author on both papers, said: "Our findings show that more babies now survive being born too soon than ever before, which is testament to the highly-skilled and dedicated staff in our neonatal services.
"But as the number of children that survive pre-term birth continues to rise, so will the number who experience disability throughout their lives. This is likely to have an impact on the demand for health, education and social care services."
Study author Professor Kate Costeloe, from Queen Mary, University of London and Homerton University Hospital, added: "This research shows that while we still have some way to go in improving the outlook for extremely premature babies, we're definitely moving in the right direction.
"The similarities between two sets of children born 11 years apart also indicate that continuing to follow the older children as they grow will give us important information about the outlook for premature babies born today and in the future.
"This will help to ensure these babies get the best possible care at birth and throughout their development."