Like any other three-year-old child, Molly has brought joy to her parents. Bright-eyed and cheerful, Molly is also deaf - and that is an issue which vexes her parents, though not for the obvious reasons. Paula Garfield, a theatre director, and her partner, Tomato Lichy, an artist and designer, are also deaf and had hoped to have a child who could not hear.

'We celebrated when we found out about Molly's deafness,' says Lichy. 'Being deaf is not about being disabled, or medically incomplete - it's about being part of a linguistic minority. We're proud, not of the medical aspect of deafness, but of the language we use and the community we live in.'

Now the couple are hoping to have a second child, one they also wish to be deaf - and that desire has brought them into a sharp confrontation with Parliament. The government's Human Fertilisation and Embryology (HFE) bill, scheduled to go through the Commons this spring, will block any attempt by couples like Garfield and Lichy to use modern medical techniques to ensure their children are deaf. The bill is a jumbo-sized piece of legislation intended to pull together all aspects of reproductive science in Britain and pave the way for UK scientists to lead the field in embryology. But in trying to do so, the civil servants drafting the bill have provoked a great deal of unrest.

'Paula is now in her early 40s,' says Lichy. 'Our first daughter was born naturally, but due to Paula's age, we may need IVF for the second.' The trouble is that, according to clause 14/4/9 of the bill, the selection of a hearing child through IVF is permitted, but embryos found to have deafness genes will be automatically discarded. 'This sends out a clear and direct message that the government thinks deaf people are better off not being born,' says Steve Emery, a sign-language expert at Heriot-Watt University.

This point is backed by Lichy. 'It is a cornerstone of modern society and law that deaf and hearing people have equal rights. If hearing people were to have the right to throw away a deaf embryo, then we as deaf people should also have the right to throw away a hearing embryo.'

Garfield and Lichy say they will continue to try for a second baby naturally and will be happy if it is able to hear. Yet their affront over the blocking of their use of IVF is intense. 'I find this shocking, detestable and utterly inhuman,' says Lichy. 'I'm a governor at a school for deaf children. When I visit, I see a class of 30 deaf children, all happily signing to each other, running about and creating chaos. Some will be artists, some will be accountants, some may go to Oxbridge, following the path that several of my deaf friends have already beaten. If they had been conceived via IVF, and detected as deaf at that stage, then all would have been aborted before birth.'

The government is unlikely to change its mind over this issue, it appears. Nevertheless, it illustrates the intense feelings that surround the embryology bill. Its passage through the Commons is set to be one of the most passionately debated in recent years. Designed to update the HFE Act of 1990, which in turn updated the Abortion Act of 1976, the new bill has been drafted to ensure the law is compatible with modern medical practice. Given the issues - from abortion to hybrid embryos - it covers, the bill was always going to stir controversy.

This point is demonstrated, somewhat unexpectedly, at Dr Warren Hern's clinic in Boulder, Colorado. Hern is one of a handful of specialists worldwide willing to perform abortions beyond 24 weeks' gestation, the legal cut-off point in most of Europe for terminating a pregnancy. And he is increasingly seeing British women for terminations that would be against the law in their home country, despite the fact that British providers - nervous of entering a legal grey area - refuse to refer them to him.

'They find me on the internet. The consequences of the refusal to refer is that they are further along, higher risk and higher cost (when I do see them),' Hern told The Observer. 'This is cruel and unusual punishment for women who need this service in issues of foetal abnormality.'

Depending on the bill's passage, his clinic could soon become a lot busier, however. Tory backbencher and former nurse Nadine Dorries is to table an amendment to the bill which would reduce the upper limit for abortions in Britain from 24 to 20 weeks: David Cameron has pledged to support it. Dorries argues there is evidence that babies above this age are sentient - capable of feeling pain - although the scientific evidence is hotly contested.

Dorries first thought there was no chance of changing the law but is now more confident: 'I only have to walk through the House of Commons and MPs say: "I am with you on 20 weeks, I don't want to go any lower, don't want to ban abortion, but I'm with you."'

Pro-choice campaigners, however, argue that the 2 per cent of abortions carried out every year after 20 weeks often involve severe abnormalities discovered only at the 20-week scan, or drastic changes in personal circumstances such as a woman being abandoned by the father. There are also frightened teenagers unable to accept they were pregnant or seek help early enough. Changing the law, they argue, would simply drive these women - if they could afford it - to fly somewhere like Colorado. 'People find ways,' says Louise Hutchins of the pro-choice pressure group Abortion Rights.

The Commons debate will concentrate on viability, the age at which a baby is considered to have a good chance of survival outside the womb. Public Health Minister Dawn Primarolo, who will steer the bill in the Commons, will tell MPs the medical consensus remains that babies cannot be considered viable below 24 weeks, which should remain the legal limit.

'The care of premature babies is clearly improving but it hasn't improved to the point where you can move the point of viability,' she told The Observer. 'There just is a certain time limit when things like lungs are formed. Clearly if the science changes we would have to make that clear to Parliament, but it hasn't.' MPs will have a free vote on abortion, a traditional issue of conscience but Labour's chief whip has rebuffed pleas from anti-abortion MPs to be given the same freedom over other controversial parts of the bill, from provisions on stem-cell therapy to animal-human hybrids.

A further issue provoking controversy has been the decision to include clauses that would require the use of all tissue used to create lines of stem cells to have the explicit consent of its donor, while another clause would block the use of any tissue from children, even if their parents give consent. This has caused considerable concern because scientists take DNA from tissue of individuals with genetic conditions, insert this into a human or animal egg cell and then create stem cells, which can be grown in laboratories. These cell lines have the same genetic defects as the patient. New therapies can then be tested on them.

The new bill, as it stands, would have a devastating impact on this kind of medical research, as leading UK scientists - including three Nobel prize winners, Sir Martin Evans, Sir Paul Nurse, and Sir John Sulston - recently warned. The law would be retrospective, so cell lines on which scientists are now working would have to be thrown out. In addition, in the case of tissue taken from children, there is the problem that many of them have conditions that mean they will not reach adulthood and, therefore, could never give consent for taking their tissue at a later date.

The issues vex scientists. To date, however, the government has refused to back down, further inflaming debate. How the bill will finally emerge from the ensuing negotiations is difficult to determine. One option being discussed is for chief whip Geoff Hoon to impose a 'soft whip', meaning Catholic cabinet ministers such as Ruth Kelly and Paul Murphy can be conveniently absent from the vote over issues conecrning abortion, rather than having to choose between their government and their consciences.

But Gordon Brown is said to be heavily personally committed to the bill, and to the cause of medical research. His second son Fraser has cystic fibrosis - one of the conditions that could ultimately be cured by stem cell therapy.