Today, as you’ll have read in many a newspaper article and on this website, MPs are voting on whether to reduce the time limit for abortion from 24 weeks. It’s a hugely important vote that has implications for us all but who is saying what, what do you need to know and why does it matter? Here’s The Lipster guide to the biggest challenge to women's rights in two decades – and join in the debate below.

Who made the vote happen? Conservative MP Nadine Dorries, who proposes that the limit be reduced from 24 weeks to 20. Dorries was the first to table an amendment to the human fertilisation and embryology bill, the main part of which was voted upon last night (which saw MPs vote to allow the creation of hybrid embryos for research). Conservative MPs Edward Leigh, Claire Curtis-Thomas and Ann Widdecombe have also proposed an amendment to bring the time limit down to 12 weeks, and there are other suggestions to bring it down to 14, 16, 18 and 22 weeks. The Guardian’s women’s editor Kira Cochrane has called this barrage of amendments "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."

What is Dorries’ campaign based upon? Not one but twenty reasons – captured snappily in the campaign title 20 Reasons For 20 Weeks. Read them all here. In them, Dorries talks about the majority of doctors performing late abortions coming from “overseas”, gives tenuous examples of women having late abortions and concentrates throughout on the rights of the foetus rather than the rights of the mother. For another perspective, read Bad Science’s dissection of Dorries’ campaign here.

Who is supporting Dorries’ campaign? There's The Daily Mail, The Daily Telegraph and – most importantly, according to political pundits – David Cameron.

Who isn’t supporting Dorries’ campaign? 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, Marie Stopes International and the British Medical Journal-endorsed EPICure study, a study of 650,000 births between 1994 and 2005 that revealed that there had been no improvements in survival rates for prematurely-born babies despite advances in science.

What is the most worrying thing about this vote? Apart from its premise having no link with scientific evidence, and its use an emotive tool in the Crewe and Nantwich by-election pre-amble, everything boils down to this: the way in which the debate concentrates on the rights of the foetus over the rights of the mother. As Kira Cochrane put it in yesterday’s Guardian:

"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… women who, by their own admission, are saying, 'I cannot cope with having this child.' “

What can you do? If you live in London, the campaign group, Abortion Rights, is staging a protest outside Parliament tonight. If you can’t or live elsewhere, you can get in touch with your MP last-minute by emailing them through the WriteToThem website, which you can access here. This action may seem like a late stab in the dark, but as pro-choice MP Emily Thornberry put it perfectly in yesterday's Guardian article, “our first principle has to be to defend what we have”. So let's start defending.