The Prime Minister is embattled, the Chancellor has torn up the Budget and the Labour Party is set to lose a seat it has held since the days of Michael Foot.

Yet early next week MPs will briefly forget about 10p tax, public borrowing rules and even the Crewe & Nantwich by-election as party hostilities are suspended to consider different controversies.

The House of Commons will have its first chance in years to vote on matters of reproductive medicine, stem cell research and abortion as the Human Fertilisation and Embryology Bill enters its committee stage on Monday and Tuesday.

The issues at stake lie well outside normal political comfort zones, encompassing hybrid embyros, “saviour siblings”, fertility treatment for lesbians and the viability of the foetus.

Coalitions cut across party lines. On abortion, for example, the campaign for lower time limits is led by religious MPs such as Nadine Dorries, a Conservative, and Claire Curtis-Thomas, of Labour. Support for the status quo is being marshalled by Evan Harris, a Liberal Democrat, Jacqui Lait, a Tory, and Alan Milburn, the former Labour Cabinet minister. The Times here presents a guide to the four three-hour debates, each of which will end in a free vote.

Human-animal embryos

Monday's first debate will cover the issue that has received most attention: the creation of “human admixed embryos” containing human and animal material.

These have been condemned by Cardinal Keith O'Brien, head of the Roman Catholic Church in Scotland, as “experiments of Frankenstein proportion”. Yet to most scientists and patient groups they have valuable potential for medical research.

The main type of admixed embryos are “cytoplasmic hybrids” or “cybrids”, generated by moving a human cell nucleus into an empty animal egg. The result is genetically 99.9 per cent human and could carry the DNA of a patient with a degenerative disease.

The idea is to make stem cell models of conditions such as Alzheimer's and Parkinson's to study their progress and to test new treatments. Admixed embryos would be cultured for a maximum of 14 days.

This issue is so new that only a few countries have passed legislation. Australia has banned it, Canada allows some kinds of admixed embryos but not others, and China is an enthusiastic supporter.

Two amendments have been tabled. The first, from Edward Leigh, a Conservative, would ban all these experiments, and is unlikely to be passed. Conservative and Liberal Democrat MPs who favour the research are thought to outnumber Labour MPs who do not, and the public is also supportive. A Times/Populus poll last month found that 50 per cent approve cybrids, against 30 per cent who are opposed.

A second amendment, from Andrew Lansley, the Tory health spokesman, would outlaw only true hybrids, made by fertilising a human egg with animal sperm, or vice-versa. This stands more chance of success, not least as scientists have not yet planned any experiments of this sort. Researchers say that a ban would be unwise because science can move quickly. True hybrid experiments would still have to be approved as “necessary or desirable” by the Human Fertilisation and Embryology Authority (HFEA).

Amendment prospects: **outright ban, ***ban on true hybrids

Saviour siblings

The Monday evening debate will cover the issue that, more than any other, prompted the Government to update the embryology law: the creation of “saviour siblings”.

It has been possible since 1990 to test IVF embryos for genetic diseases such as cystic fibrosis. In 2001 the technique was adapted to screen for tissue type, to let parents of sick children to conceive a sibling suitable to provide a life-saving transplant, using cells from the umbilical cord or bone marrow.

This threw the HFEA into confusion as it was not specifically covered by legislation. The Hashmi family was allowed to try to conceive a saviour sibling for their son, Zain, who has the blood disorder beta-thalassaemia, though the treatment failed. The Whitaker family, however, was refused permission to use the technology in a slightly different way and went to the US for therapy. The successful procedure cured their son Charlie, now 9, of Diamond-Blackfan anaemia. The HFEA reversed its position, leading Ian Gibson, then chairman of the Commons science and technology committee, to accuse it of “making policy by the seat of its pants”.

Critics of allowing saviour siblings say it commodifies children. Some also object to the destruction of embryos not suitable as donors. An amendment from David Burrowes, a Tory, to ban it is expected to fail.

Mr Lansley has proposed a milder amendment that would make it clear that only “regenerative tissue”, such as cord blood or bone marrow, could be taken from saviour siblings. This could pass, though the Bill already bans the removal of whole organs.

Amendment prospects: **

Need for a father

Before providing fertility treatment, at the moment clinics must consider a child's “need for a father”. The Government's proposal to replace this with “need for supportive parenting”, to avoid discrimination against single and lesbian women, will be debated on Tuesday afternoon.

Opponents of the change, such as the former Tory leader Iain Duncan Smith, have argued that research shows the absence of a father is detrimental to children. They point out that few lesbians are denied fertility treatment under current law.

Both statements are correct - in part. There is good evidence that children without fathers do worse, but it comes from studies of family breakdown. Research led by the psychologists Clare Murray and Susan Golombok shows that children of lesbian and single women who conceive by donor insemination are as well adjusted as those from traditional families.

Equality laws say that these women are entitled to treatment. The HFEA code of practice tells clinics to consider “the prospective mother's ability to meet the child's needs” in such cases.

Lawyers and gay rights groups say that fertility clinics still discriminate against lesbians. A decision to reinsert the father clause would worsen this because Parliament would have made its opinion clear.

Two amendments are on the table. Mr Duncan Smith's would enshrine in law the “need for a father and a mother”. Mr Lansley's proposes a need for a father or male role model”. One of these is likely to be adopted.

The Conservatives are encouraging their MPs to back the amendments, which also have some Labour support. It is possible that any amendment would be struck down by a legal challenge under the Human Rights Act.

Amendment prospects: *****

Abortion

The most controversial debate will be the last. Although the Bill does not mention abortion, the Government has allowed time for votes on the issue on Tuesday evening and nine amendments to current law have been tabled.

Mrs Dorries has proposed cutting the current 24-week limit to 20 weeks and other MPs have tabled alternatives of 12, 14, 16, 18 and 22 weeks. Only some are likely to be put to a vote. Other proposals include a compulsory cooling-off period before an abortion could be carried out.

These amendments are supported by MPs who oppose abortion in principle and by others who cite evidence that foetuses are sometimes viable at 23 or even 22 weeks.

MPs who want to restrict abortion rights claim that most European countries have lower limits, often at 12 weeks, as in Germany, Italy and Spain. These generally apply to women's absolute right to abortion: later abortions are still permitted with medical permission, as in Britain.

MPs who back the status quo, such as Dr Harris and Ms Lait, point to two recent studies which show that survival among babies born at 22 and 23 weeks has not improved for a decade.

Late abortions are rare: 89 per cent are performed before 13 weeks, fewer than 2 per cent are after 20 weeks, and 0.7 per cent are after 22 weeks. They tend to feature difficult cases. This is the hardest of all the votes to predict, though it is unlikely that a reduction to less than 20 weeks will be carried.

GLOSSARY

Human admixed embryo
Embryo containing both human and animal material

Cytoplasmic hybrid embryo, or “cybrid”
Cloned embryo made by placing human nucleus into an empty animal egg

True hybrid embryo
Made by fertilising human egg with animal sperm, or vice-versa

Chimera
Admixed embryo, containing a mixture of human and animal cells

Transgenic human embryo
A genetically modified human embryo, containing animal DNA

Pre-implantation genetic diagnosis (PGD)
Embryo-screening technique

Saviour sibling
Child born after PGD test to establish suitability as tissue donor

Need for a father
Clause that requires IVF clinics to consider a child's need for a father

Abortion time limit
This is 24 weeks. Amendments have been tabled to reduce it

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