There is no evidence to support a change in the law on abortion, a health minister said.
Dawn Primarolo told MPs the 24-week upper limit on abortion was a matter to be debated in the House of Commons.
But she said the medical consensus showed that, although there had been improvements in care in recent years, a foetus's chance of survival had not improved.
At 21 weeks gestation, the chance of survival to discharge from hospital was 0%, at 22 weeks it was 1% and at 23 weeks it was 11%, she said.
The term "viability" is controversial, with some arguing it is synonymous with being born alive, irrespective of the length of time the baby survives.
Others believe it should refer to the baby's ability to have a meaningful life.
Ms Primarolo told the Commons Science and Technology Committee: "In this very complex area of viability we are following the medical consensus and that medical consensus still indicates that, while improvements have been made in care, at the moment that concept of viability cannot be pushed back in (terms of) weeks.
"It's a matter of viability and survival rates and that's where we draw our view on that matter."
Ms Primarolo said lowering the age definition for viability would imply that viability and survival rates had improved, and "they have not in the consensus of evidence we have."
She continued: "Certainly the evidence with regard to the British Association of Perinatal Medicine is pointing quite clearly to the point that viability, whatever the vast improvements, and there are significant improvements, cannot be continually pushed back in terms of a date."
