There has been no improvement in the survival rate of babies born before Britain's 24-week legal limit for abortion, doctors said on Friday, ahead of an expected attempt by pro-life legislators to reduce the cut-off period.
A study of 650,000 births between 1994 and 2005 in the Midlands showed clear improvements in survival rates for infants born after 24 and 25 weeks' gestation.
But the research, published in the British Medical Journal, found survival rates for babies born at 22 and 23 weeks had not changed during the 12 years covered.
David Field, professor of neonatal medicine at Leicester University and lead author of the study, said medical advances that had aided older babies had failed to assist the younger ones.
"Doing exactly the same thing for these more immature babies doesn't seem to have made any difference at all," he said.
"It's as if there is some maturity effect that kicks in around 23 and 24 weeks."
The study was based on records of births in the former Trent health region, covering Derbyshire, Lincolnshire and Nottinghamshire.
In the 12 years to 2005, all 150 infants born alive at 22 weeks eventually died.
Pro-life MPs are expected to submit amendments to the Human Fertilization and Embryology Bill on Monday calling for reductions in the 24-week abortion limit on the grounds that a greater number of very immature babies now survive.
But Field said such claims were distorted by taking data from specialist neonatal hospitals which tended to over estimate the likelihood of survival.
"One of the reasons we produced this paper is we felt there had been some misinformation about the situation," he said.
"If you report data from a single unit, or two or three particular hospitals, you can get rather spurious results."
Last October, the House of Commons Science and Technology Committee said there was no scientific basis for lowering the limit for legal abortions.
MPs are traditionally given a free vote on abortion matters in the House of Commons, and reduced the limit to 24 weeks from 28 weeks in 1990.
Conservative Leader David Cameron has said he would support a further reduction, while Prime Minister Gordon Brown's spokesman has said the premier would not vote for a change.
The government has said it has no plans to alter the existing limit.