Haemadipsa picta, the Borneo tiger leech, sits on a leaf in the rainforest, waiting for something, anything to pass by: an orang-utan, you, my daughter Eve, whatever. Actually it sort of stands there like a tiny penis, using its horrid thin end as an antenna, trying to detect body heat. When it does, boing! It throws itself off the leaf and down your Calvin Kleins, there to suck your blood. I've seen it do this, but most people really don't believe it until it happens to them.

The tiger leech, however, is a monstrously inefficient parasite compared with Britain's anti-abortion movement, which - always alert to the least possibility - has somehow leapt from its bush on to the Human Fertilisation and Embryology Bill, and may by this evening have won an improbable vote to curb the abortion rights of British women. While most of us were distracted, engaged in debating saviour siblings and hybrid embryos, Haemadipsa Restricta worked its way through our political underwear.

Those of us who support the status quo on abortion rights have been complacent. Specifically we should have realised just how tempting, white, soft, male and vulnerable were the parts belonging to the young men around David Cameron. They could support a “compromise” over abortion, which would reduce the number of weeks within which a termination could be carried out, but not reduce by as much as the anti-abortionists wanted. That way these new Tories could get the plaudits of the social authoritarians without losing the patina of being right down there with the kids.

So Andrew Lansley, the agreeable Shadow Health Secretary, favours a reduction to 22 weeks and David Cameron and urbane young hipsters like Ed Vaizey, MP, may support going as low as 20 weeks. And it was all there, had we been wearing our spectacles.

In February 2005, when Mr Cameron was, in leadership terms, pre-zygotic, he was asked about abortion by Sir David Frost. “I think the 24-week limit does need to be looked at,” explained Mr Cameron, “because of advances in medical science, and because I think the debate has moved on.” Earlier this year he elaborated: “I would like to see a reduction in the current limit, as it is clear that, due to medical advancements, many babies are surviving at 24 weeks.”

This is it - medical advance and the debate moving on. Moving debates are a poor reason for making life-and-death changes in the law governing personal responsibility, but medical advance should be another matter. Is there significant evidence that the foetus is now significantly more viable at up to 24 weeks than was the case in 1967 or 1990, when the law was last changed?

“No” is the answer to that. Camera technology has certainly advanced, and we can capture the foetus in the womb, and from the images sentimentally imagine that we know its “thoughts” or “feelings”, but the latest study has established that the survival rates for severely premature babies have not improved over the past 18 years. Hardly a single baby born at 22 weeks or under manages to leave hospital alive, and at 23 weeks 82 per cent fail to make it outside, the same percentage as in the mid-1990s.

In contrast to Mr Cameron's invocation of scientific advance, Professor David Field, president of the British Association of Perinatal Medicine and lead author of this most recent report, concluded that the compared figures were “almost as identical as you can get it. There is no change.” The various royal colleges all agree.

Given these facts, one is entitled to ask just what on earth is going on here? If viability isn't the test - as it was claimed to be back in 1990 when the limit was reduced from 28 weeks - then the judgment must be that some folk simply don't like abortions and wish to restrict them as much as possible. Or as much as they can get away with.

Enter, with a clash of brass, from the direction of Mid-Bedfordshire, the Tory MP Nadine Dorries, “former nurse” as she is invariably described. Her 20-week campaign, bounced from its tree by the passing debate, scorns Mr Lansley's 22 weeks because, as she explains: “Cutting the limit to 20 weeks will save thousands of lives, but to 22 weeks, only hundreds.” Or, to put it another way - despite the scare stories - very few abortions are carried out between 22 and 24 weeks, but there are more at 20.

Ms Dorries's favourite prop is a picture of an operation on a “baby Samuel” - little hand and all - being carried out in utero, accompanied by the claim that, “if the 22-week amendment is passed, babies just like him will continue to be chopped up. I know it's horrible and it's not language I like to use believe me - but it's a fact.” In my experience when someone says they hate to say something, they mean the exact opposite.

Of course, what is true of 22 weeks in Dorriesland is true in spades for, say, 16 weeks, or 12 weeks. The lower you go then, in Dorriespeak, the more lives you save. So 20 weeks is clearly nothing more than an arbitrary device designed to maximise support from those who feel queasy about abortion, or pressured by anti-abortionists but who can't be bothered to look at either the science or the implications. It is an exercise in damaging hypocrisy.

Nor can there be the facile trade-off that some suggest, with easier abortion at an earlier stage - desirable though this may be. One of the overwhelming reasons for late abortion is the failure by the woman (or, as often happens, young girl) to realise that she is pregnant. Occasionally an older woman mistakes the end of menstruation for the menopause. Or perhaps the lifestyle of the woman is simply chaotic. All we do know for sure is that this person has decided that she doesn't want to take her pregnancy to term.

So this is what all this nonsense about “compromise” boils down to - telling women who are less than 24 weeks pregnant and who don't want to have a baby that, legally, they must go through with the birth. We then leave them the terrible choice of procuring an abortion elsewhere or of bearing a child they do not want.

To me, this is immoral. It is not a conjecture about lives that could be led, but an action that will damage lives that are being led. Tonight conscientious MPs should put on their leech-socks and vote against all these parasitical amendments.

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