A health minister has told MPs that there is no evidence to support a change in the legislation governing abortion.
The 24-week upper limit on abortion is a matter that needs to be debated in the House of Commons, Dawn Primarolo said.
But she added that the medical consensus shows that, although there have been improvements in care in recent years, a foetus's chance of survival has not improved.
She said that at 21 weeks, the chance of survival to discharge from hospital is 0%, at 22 weeks it is 1% and at 23 weeks it is 11%.
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 have improved, and "they have not in the consensus of evidence we have."