The key question: when is the foetus a sentient being?

One medical question lies at the heart of the debate over the abortion time limit - when does a foetus become a sentient being?

One medical question lies at the heart of the debate over the abortion time limit - when does a foetus become a sentient being?

The publication of a book of extraordinary photographs of babies in the womb, using a 3D scanning technique, dramatically reopened the issue.

The book, Watch Me Grow, by Professor Stuart Campbell, former head of obstetrics at the King's College Hospital, London, captured pictures of babies at 12 weeks "jumping off the sides of the womb like a trampoline," opening their eyes at 18 weeks and apparently smiling at 22 weeks.

Medical and lay opinion has since grown in favour of a reduction in the time limit for abortion. But pro-choice organisations such as the British Pregnancy Advisory Service (BPAS) and the Family Planning Association warn that will close the options for the most vulnerable women, often teenagers unaware they were pregnant or mothers who discover too late their foetus has an abnormality.

Ann Furedi, the chief executive of BPAS, said: "We think the law is best left alone. It works reasonably well. You can focus on abortion as a political issue and forget about the very difficult circumstances women find themselves in. The real scandal, is women cannot get late abortions because there are not enough appointments available."

Professor Campbell insisted he was "pro-women and pro-choice" but said he was in favour of reducing the time limit from 24 to 20 weeks.

"You have got to be human to the woman but you have got to be human to the foetus, too. I don't think we could have a big change but we are at a point where we can detect abnormalities sooner than we used to. It doesn't seem unrealistic to think we could bring back the limit."

Officially, medical organisations such as the British Medical Association and the Royal College of Obstetricians take no view on whether the 24-week limit should be reduced, leaving it to Parliament to decide. A spokeswoman for the BMA said: "There are no problems with the act - it is a practical and humane piece of legislation. We are not pressing for a reduction in the time limit."

But privately, doctors and nurses, like their women patients, dislike late abortions and many decline to do them.

Peter Bowen Simpkins, a consultant obstetrician at Singleton Hospital, Swansea, said: "Most NHS hospitals stop at 18 weeks. The nurses - and the doctors - don't want to look after patients beyond that limit. I can't speak for every hospital but late abortion is not a very pleasant affair. They are mostly done in the private sector. Many doctors and nurses don't do abortions at all."

Abortion was legalised in the 1960s to end the scourge of back-street terminations that caused the deaths of at least 50 women a year from botched surgery and infection.

The 1967 Abortion Act, introduced by the Liberal MP David Steel, set an upper time limit of 28 weeks based on the assumption in the Infant Life Preservation Act of 1929 that this was the limit of viability - the minimum age at which a foetus could survive.

No time limit was set in Scotland, yet no more late abortions were performed north of the border than in England, a fact cited by pro-choice campaigners as evidence that the law does not prevent late abortions. Women and their doctors do, because neither want them.

The 1967 Act came under repeated assault from anti-abortion organisations to reduce the time limit and to repeal the law in Parliament. They did not succeed until 1990, when the abortion act was reviewed as the Human Fertilisation and Embryology Act was going through Parliament and the time limit was reduced to 24 weeks.

Advances in medicine have seen babies survive at 23 weeks and, very rarely, 22 weeks. But the poor outcome for those suggests the limits of viability have been reached.

WHERE LEADERS STAND

Tony Blair , who entered Parliament in 1983, has consistently supported the Abortion Act introduced in 1967. In 1988, Mr Blair opposed David Alton's Abortion (Amendment) Bill which aimed to lower the upper limit for abortions from 28 weeks to 18 weeks. In three votes, he voted against.

In 1990, he voted for a clause aimed at permitting abortion on the agreement of only one doctor if the pregnancy had not exceeded 12 weeks. He opposed pro-life amendments to ban abortions after 20 weeks and after 22 weeks.

Michael Howard , Conservative leader, who also entered Parliament in 1983, has voted for tightening the law on abortion. He supported the David Alton Abortion (Amendment) Bill which aimed to lower the upper limit for abortion.

In 1990, he opposed attempts to legalise abortion on demand with only one doctor needed to certify that the pregnancy has not exceeded 12 weeks. He also voted against abortion up to birth on various grounds including handicap. He voted against the Abortion Act to be extended to Northern Ireland.

Charles Kennedy , Liberal Democrat leader, has supported the "pro-life" lobby, opposing abortion on demand and late abortions. He voted for a time limit of 22 weeks in the past but said last week: "I don't know what I would do now". Anti-abortion campaigners say he has rarely attended their debates on the issue.

Mr Kennedy believes abortion should remain an issue of conscience for MPs, not dictated by party whips, although it is unclear whether he would favour devoting Government time to legislation to reform the situation on abortion.