In the end, science carried the day. MPs decided not to break the powerful link between foetal viability — the age at which a premature baby can survive — and the law.

That was enshrined in the original Bill in 1967, when viability was presumed at 28 weeks. In 1990, when the issue was debated again, new ventilation techniques and the use of steroids meant that an increasing number of premature babies were surviving, so the limit was reduced to 24 weeks.

This time medical experts made clear that there had been no improvement in viability at 22 and 23 weeks since 1990 and called for the limit to remain at 24 weeks. It was a powerful argument made by powerful institutions, including the British Medical Association, the Royal College of Obstetricians and Gynaecologists, the Royal College of Nurses and the British Association of Perinatal Medicine.

If you sever the link between the law and the science, where do you draw the line?

It looked like a comfortable victory for the Government and pro-abortion campaigners. They had expected it to be a far closer affair.

Dawn Primarolo, the Health Minister, said that the upper limit was set by Parliament in 1990 at 24 weeks because scientific evidence at the time showed that the threshold of viability had increased. “It has always been linked to the potential viability of the foetus outside of the womb. That was the case in 1967. It was the case in 1990 and certainly the case now.”

But the anti-abortion campaigners will not go away. A hard core of 80 or so MPs voted for every restriction that was on offer last night. A larger group of 233 supported a reduction in the limit to 22 weeks. They are clearly uneasy about how the current system is working.

Much of the talk during last night’s impassioned debate was about foetal viability. The recently published Trent study found that no babies survived at 22 weeks and that only 18 per cent of those born at 23 weeks left hospital. That 18 per cent also encompasses only those admitted to neonatal units — include those who were stillborn or who died in the delivery room, and viability looks even worse.

These survival rates, which match those from a similar recent study called Epicure 2, have not improved in 10 years — though survival at 24 and 25 weeks has increased markedly over the same period. This has led Professor David Field, who led the Trent study, to argue that the limits of foetal viability have been reached.

However, the small number of abortions performed between 22 and 24 weeks were not the primary target of anti-abortion campaigners. Women who have these late abortions are often among the most unfortunate cases. Some are schoolgirls who deny that they are pregnant until they can fool themselves no longer.

Others, the largest group, do not realise that they are pregnant for up to three or four months and are then subjected to delays before they can have an abortion. Few primary care trusts perform abortions beyond 16 weeks, so they have to hunt around for slots at the small number of clinics which do.

Another smaller group will have been diagnosed with foetal abnormalities at their 20-week scan. What has galvanised wider support for more restrictions on abortion during this debate is the inexorable rise in the number of terminations taking place in Britain — 193,700 last year compared with 177,495 a decade ago. In the 1970s the number was an average 120,000 a year.

There is considerable unease about the steady upward march in the figures, even among the staunchly pro-abortion lobby.

Last year Marie Stopes International, the biggest provider of abortions outside the NHS, revealed that it had carried out more abortions in January than at any time in its 32-year history and appealed to women to take more precautions.

Another concern is the number of multiple abortions. Figures show that, in 2006, 15,000 women who had terminations were on their third abortion and about 3,800 were on their fourth.

There is little that the anti-abortion campaigners can do to cut the overall numbers, since 89 per cent take place before 12 weeks, or the incidence of multiple abortions. Unlike in the US, in Britain there is no “provisional wing” of the anti-abortion movement that threatens to blow up clinics or harass the doctors who work there.

They will be heartened by David Cameron’s decision to back a reduction to 20 weeks. Dozens more Conservative MPs backed a cut to 22 weeks.

If the Conservatives form the next government, there is likely to be another chance to bring the issue to a vote within a few years, rather than wait for another 20 years. And if the number of abortions continues to rise, they will have even more ammunition next time.

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