Ostensibly, the Human Fertilisation and Embryology Bill that returns to the House of Commons this week says nothing new on abortion. In practice a series of amendments to the Bill represents the most serious challenge since 1990 to the 24-week upper legal limit for abortions. Most of these amendments claim scientific backing. They also reflect a growing unease with a steady increase in the number of abortions in the UK and a perception that the practice - legalised in 1968 to end dangerous and degrading back-street abortions - has evolved since then into a lifestyle choice for the selfish and irresponsible.

The Commons debate will be impassioned. Just as surely, the fundamental arguments over when a foetus is sentient, “viable” and in possession of rights to be balanced against its mother's, will remain unresolved. It is right, therefore, that the vote on the Bill will be free, and it is likely, as a result, that the view of Parliament on this troubling subject will be complex. Yet this much is already clear: the basis for the most popular amendment to the current law is flimsy. Furthermore, the debate on foetal viability, though it may take up parliamentary time, is a proxy for a broader debate whose parameters have changed little since the time limit for abortions was last changed 18 years ago. For these reasons, the existing statute deserves to survive unchanged.

Happily, huge strides have been made in neo-natal medicine in the past generation. Two thirds of babies born in Britain at 25 weeks are now expected to survive. There have been isolated cases of even more severely premature infants surviving to live healthy lives, and in certain specialist neo-natal units survival rates for births at 22 and 23 weeks have increased since the mid-1990s. On this basis a substantial minority of MPs is calling for a reduction of the upper legal limit for abortions to 20 weeks. But the data they cite is exceptional and too easily misleading.

A study of pre-term infants' survival rates published last week in the British Medical Journal found no significant improvement in the past 18 years for those born earlier than 24 weeks, despite marked improvements for those born at 25 weeks and later. As a survey of a “whole” population (in the East Midlands), the study is a better basis for legislation - or lack of it - than the single-hospital studies preferred by backers of the 20-week amendment. It casts serious doubt on the claim made by Nadine Dorries and others that lowering the legal limit this far would save hundreds of infants' lives. And, as Dawn Primarolo argues in an interview with The Times today, it suggests that lowering the limit would raise unfounded hopes for parents of extremely premature babies with little real chance of survival.

Insofar as science is a guide to this debate, the new study also undermines the case for a 22-week amendment, and Mrs Dorries is right to criticise Labour MPs thinking of supporting it as a political compromise. The law on abortion is no subject for politicking, as Westminster has hitherto accepted to its credit. It is a reflection of society's evolving assessment of the balance between a woman's right to choose and a more general obligation to preserve life and avoid the unique trauma of late-term abortion. Medical science may yet justify lowering the 24-week limit. It has not done so yet. In the meantime, all sides should be thankful that 90 per cent of abortions happen in the first trimester. They should also work together to make that proportion even higher and the total number of abortions lower.