Jamie Swanborough is a boisterous 2½-year-old who can be a real handful, according to his mother Catherine, a former sales consultant. In his home town of Southampton there is little to distinguish this healthy toddler from the dozens of other children in their pushchairs round the town centre.

The difference is that when Jamie was born on October 16, 2002, Catherine was only 23 weeks pregnant — which means that he was born below the legal limit at which abortions can be carried out. He weighed 1lb 6oz, was given only a 5% chance of survival and spent five months in hospital.

“It makes my blood run cold when I look at him running around and talking and realise that I could have had an abortion. He is just perfect,” she says.

A few years ago Jamie’s survival would have been unthinkable; and even now the prognosis of those born at 23 weeks is exceptionally poor (only 17% survive and three-quarters of those will suffer long-term disability). But the fact that any of these babies are being kept alive by medical science has once again raised the question over when a foetus becomes a sentient being — and how we should define the law on abortion.

While 30 years ago women fought passionately to end the horror of back street abortions, which caused the deaths of 50 women a year, today many women feel disturbed about late abortions taking place in one hospital room, while down the corridor doctors are fighting to save the life of a premature baby.

“When I was in my early twenties I was very idealistic about abortion as a woman’s right,” says Kate Warren, 32. “But after seeing my friends have children and having had to accompany a friend to a south London clinic where there were these desperately scared teenagers with nobody to support them, I have felt increasingly uneasy about late abortion.”

Of course the rights and wrongs of abortion have been debated ever since the 1967 Abortion Act legalised terminations. But unlike in the United States, where political leaders have to proclaim whether they are pro-choice or pro-life, British politicians have usually escaped such scrutiny. Yet the three party leaders have now been quizzed by Cosmopolitan magazine on where they stand. While Tony Blair says there are no plans to change the law and Charles Kennedy says that he has voted for a 22-week limit in the past, Michael Howard claims that women are effectively able to get terminations “on demand” and that the legal limit should be lowered to 20 weeks.

“This is political fundamentalism,” says Rosalind Miles, author of The Women’s History of the World. “I cannot understand why these men are once again trying to control women in this way.”

What really ignited the debate, however, was the intervention a day later of Cardinal Cormac Murphy-O’Connor, Britain’s most senior Catholic churchman. In a letter published last Monday he commended Howard’s stand and admitted that it could mean a historic break with the Catholic church’s traditional backing of the Labour party. “Abortion, for Catholics, is a very key issue — we are totally opposed to it,” he said.

The 1967 act permitted abortion up to 28 weeks with the agreement of two doctors if the mental or physical health of the mother could be endangered by continuing the pregnancy. Concern that a 28-week foetus was capable of survival outside the womb led to a change in 1990 when abortion for so-called “social reasons” was limited to 24 weeks’ gestation.

The publication of ultrasound pictures last year by Professor Stuart Campbell, former head obstetrician at King’s College hospital, London, made many question whether the limit should be lowered still further. Campbell’s images showed the foetus apparently bouncing on the wall of the womb at 12 weeks, opening its eyes at 18 weeks and smiling at 22 weeks. While some ridiculed these as sentimental interpretations, others felt that denying instinctive compassion for the unborn was equally unhelpful.

“There’s nothing magical about the limit of 24 weeks,” says Dr Maggie Blott, consultant obstetrician in maternal medicine at the Royal Victoria hospital, Newcastle. “It was chosen as an artificial limit simply because of the viability of the foetus. But if a baby is 23 weeks and six days you can terminate. It seems bizarre that you can be fighting to save a baby or abort one when they are the same age.”

The real controversy is that the 1990 act allowed abortion up to birth in cases of foetal “handicap”. This has never been defined and those who oppose the law claim that some abortions happen unnecessarily late and for reasons that are unjustified. This was the argument of Joanna Jepson, the curate who had tried to get two doctors prosecuted after she learnt they had carried out an abortion at 28 weeks on a foetus with a cleft palate. Jepson, who was born with a jaw defect, had argued that the foetus was suffering from what could be a correctable condition.