When I meet the Conservative MP Nadine Dorries she is clearly in triumphal mood, striding in late, complaining that her "tongue is sticking to the top of my mouth because I've done so many interviews". She immediately breaks off to take a five-minute phone call. She is a woman on a mission, her blue eyes intense when we finally get to talk. The subject is late-term abortion, and specifically her current drive to bring the time limit for procedures down from 24 weeks to 20. She has tabled an amendment to the human fertilisation and embryology bill to this affect, on which MPs will have a free vote in the House of Commons tomorrow. When I ask what result she is expecting, she forcefully replies: "a win. Well over 200 MPs are supporting it." Most significantly, David Cameron is among them.

Dorries has been waging a high-profile campaign - 20 Reasons for 20 Weeks - that has been supported by both the Daily Mail and the Daily Telegraph. As its name suggests, the campaign is a long list of reasons for a reduction in the time limit on abortion, many of them highly contentious. She suggests, for instance, that there are "inevitably financial vested interests" in keeping a longer time limit, and suggests that only foreign doctors will perform late-term abortions: "almost all doctors performing late abortions in the UK, in British Pregnancy Advisory Service (BPAS) clinics, are from overseas". Many of her main points hinge on viability; so, for example, she mentions baldly that there have been "high-profile cases of babies surviving well below 24 weeks", which is true, but a) is arguing on the basis of highly unusual cases (a recent report found that of those born at 23 weeks, fewer than 10% left hospital and many were severely impaired), and b) isn't the point of this debate. The point is that a woman has the right to control her own body. The authorative and definitive report, EPICure2, published just last month, and produced by a study group of neonatal experts, found that there had been no significant improvement whatsoever in the survival rates of babies born before 24 weeks in the past 10 years.

I ask Dorries why she thinks that the British Medical Association, the Royal College of Obstetrics and Gynaecology, the Royal College of Nursing and the House of Commons Science and Technology Committee support the current limit, and she immediately unleashes a rant about the way in which the BMA reached their decision, saying that doctors weren't given a proper chance to vote for a reduction. I call the BMA, whose spokesperson reads out the motion from their annual conference in 2005: "this meeting holds that the upper limit for legal abortion should be reduced in the light of new evidence of foetal developments and advances in neonatal care". That motion was roundly rejected.

Over the past few years, Dorries has become the public face of the anti-abortion movement in parliament, but she is by no means the only MP to have tabled an anti-choice amendment. In fact, a raft of them will be voted on tomorrow, in the biggest challenge to abortion rights in almost two decades - a multi-pronged attack that, aside from Dorries' proposed amendment, is taking place almost without comment. So, for instance, MPs Edward Leigh (Conservative), Claire Curtis-Thomas (Labour) and Ann Widdecombe (Conservative) have tabled an amendment to bring the time limit down to 12 weeks. There are also separate proposals, all being voted on tomorrow, to bring it down to 14 weeks, 16 weeks, 18 weeks and 22 weeks. It's a classic tactic - a range of proposals are put forward, some extreme, others apparently less so, and suddenly a vote for a reduction to 20 weeks looks like a brilliant, liberal solution. "The point is to confuse people," says Anne Quesney, head of advocacy at Marie Stopes International. "You focus attention on 12 weeks and 14 weeks - which are not likely to be attainable - and because parliament is a place of compromise, 20 weeks suddenly sounds reasonable, and 22 weeks sounds even more reasonable ... It's really quite worrying, because although the government is strong on the 24-week limit, there's also this sense that the Labour government is running scared. Labour MPs who have got small majorities: will they vote, will they abstain?" The Crewe and Nantwich by-election is on Thursday, which gives MPs a good cover for any absence - they can simply say that they were out of London, campaigning. "That's what we're worried about," says Quesney, "that people will not be in the house to vote, and stand up and be counted. The government doesn't want to lose this, because I think it would look bad for them, but they want to keep as low a profile on it as possible."

The pro-choice Labour MP Emily Thornberry is also worried. The range of proposals "is immensely confusing for MPs," she says, "and particularly when people are not used to voting without whips ... members of parliament don't necessarily want to engage with this issue, because it's highly emotive, you get some very nasty letters, so why would you get involved? What I'm fearful of is that, because the arguments have been put forward on the basis of viability, MPs who have not had this issue as their primary focus have heard all the noise, they haven't stopped to see that it's a load of nonsense, and that all the medical groups unanimously state that viability has not changed since 1990 [when the limit was reduced from 28 weeks to 24], and so they think, 'There must be something in it, I'm sure I heard of a baby that survived at 22 weeks, so therefore let's play safe, let's cut it back for a couple of weeks' ... I was given a list of generally pro-choice, but soft pro-choice people to speak to, and I was very alarmed by how soft they are."

A 20-week limit may sound perfectly reasonable; a good compromise, in fact. After all, late-term abortion is a highly emotive, divisive subject, which is exactly why the anti-abortion lobby focuses on it - narrow the debate down to late-term abortion (after 20 weeks), a procedure that makes up only 1.45% of all terminations, and you very effectively skew the argument, and undermine women's right to choose in general.

There are several reasons to be very worried about the current campaign. One is that behind many of the proposals for reductions of a few weeks seem to lie long-term ambitions for much more stringent reductions. While Dorries tells me that she is committed to the notion of a 20-week limit, she admits that, "I think what I would like to see is the European average of 13 weeks." In the comments section of an article that she wrote for the Spectator website last October, someone going by the name Nadine Dorries remarked that, "I do want to go lower than 20 weeks - I would settle for the European average of 13 weeks, but would prefer nine." In that same comment, she says that she does "not stand at zero weeks", and she emphasises to me that she is neither pro-choice nor anti-abortion, yet my suspicions are raised when she refers to a late-term abortion that she recently observed as "murder" and refers to abortion in general as "taking a life" (when I challenge her on this she says that she meant "taking a potential life, a life, or a potential life"). As our conversation continues I begin to find her position more and more disturbing. Does she really support abortion up to 13 weeks, or nine weeks, whichever it is? Or does she see abortion as morally abhorrent - "taking a life" - under any circumstances? I'm none the wiser.

Over the past few years, it has been incredible to watch how women's voices and experiences have been excluded from the mainstream abortion debate, replaced by 4D images of foetuses - which are clearly emotive, but don't change the argument - and comments such as this, from Dorries' campaign, that "babies are now undergoing surgery in the womb under 24 weeks". That sounds great, I'm very glad about it - it is also completely irrelevant. As one woman said to me, it now seems that the argument runs, specifically, "the foetus, the science, and finally the woman". Dorries tells me that she has spoken to about 200 women who have had abortions (as a side note, she says that every single one "felt that she was coerced by somebody into her abortion, whether it was a partner, a parent, a teacher", which is unlike the experience of anyone I've ever known), and so I am surprised by her reply when I ask how many women she has spoken to who have had late-term abortions. "I haven't spoken to that many," she says, "apart from on radio chat shows, that kind of thing".

It's a shame she didn't talk to more women who have had late-term abortions - if you had tabled an amendment which could significantly change the course of someone's life, wouldn't you seek out the stories of those it would affect? With the way in which the late-term abortion debate is framed by politicians and the media, it is understandable to balk at the subject, understandable to find it problematic - until you begin to speak to women who have gone through the procedure later than 20 weeks. Many of those who have late-term abortions are the most vulnerable: teenagers who didn't realise that they were pregnant until five months' gestation; women with learning disabilities; those using methadone in drug rehabilitation programmes, which puts a halt to your periods. Women like the one I read of recently, whose partner started beating her up when she became pregnant, and who feared she would never be able to escape him if she had his baby. (In more than 30% of domestic violence cases, the abuse started during pregnancy.) Women who have suffered a severely traumatic episode - the death of a partner, or a child, for instance - who fear that the stress might affect foetal development. The BPAS has just published a 28-day audit of late-term abortion requests, to be distributed to MPs. The stories include that of a woman with two small daughters from a previous marriage, who had an unplanned pregnancy with her current partner, which he urged her to continue. She then found out that he was abusing her daughters. As Ann Furedi of BPAS says, the stories "provide a really stark contrast to the abstract, philosophical and rather sterile discussion about viability and not viability. What this does is to take it woman by woman. The challenge that we're putting to MPs is to look at this and think about it - what makes you think that the lives of these women would have been better if they'd had to continue their pregnancy? We're talking about women who, by their own admission, are saying, 'I cannot cope with having this child'."

When I ask Dorries why she thinks women have late-term abortions, she boils it down to "procrastination ... when someone goes past a 12-week barrier and they're still thinking about whether they're going to or not, there seems to be an element of procrastination that comes into that.

"Everyone looks on terminations as this life-liberating thing that women go through," she says. I ask who has described abortion specifically as "life-liberating" to her, and she says "Oh God, well, a lot of the pro-choicers who I argue with do. They say, 'Get your hands out of my uterus, women have fought for this liberation for years'." She laughs for the first time in the interview.

I am feeling extremely bleak, but can't help myself: what does she think of feminism? "I don't even know what feminism is, to be honest with you. You know; what is it now? We're in a post-feminist age: what has been achieved for women now? We're still not on equal pay, in many areas of life." But we're much further towards equal pay than we've ever been. "Possibly, possibly," she says, clearly unconvinced. "Would we have been anyway though, now that we have so many more women getting into senior positions?" (I don't even bother to query this last point, or the fact that it might just have something to do with feminism itself.) "I don't know. The feminist discussion is just not something that I major in. But I know personally, I look at my daughters who pay half when they go out [on a date] with someone and I think, 'They never did that in my day'. I'm purely selfish about this, to be honest with you." This is the main insight that she makes on feminism: outrage that her daughters have to go dutch.

Of course, we all hope that we will never have to have an abortion - let alone a late-term abortion - but any woman could find herself in that situation; one in three women has an abortion, and 76% of the population support that right. As Quesney says, "It's real women we're talking about, not just case studies. This is an issue that has defined women's equality with men - if you take away abortion rights, or family planning, you take away the key to women's equality with men. It's that important, and people are about to compromise it." The campaign group, Abortion Rights, is staging a protest outside Parliament as the MPs go in to vote tomorrow night; there's also the option of writing a last-minute letter to your MP, countering what Thornberry calls the anti-abortion "background chatter" that has been influencing MPs "without them even realising that they're being influenced.

"People don't want to think about abortion," says Thornberry. "The woman or the man on the street doesn't want to think about abortion. They may think that women have a right to choose, and they may be perfectly happy with that, but they don't really want to engage, because it's a horrible subject. And then the crisis appears for them. A very large number of women find that they need an abortion. Our first principle has to be to defend what we have".