A unique legal case this week raises troubling questions about the ethics of antenatal screening and how far prospective parents are prepared to go to achieve “the perfect baby”.

Joanna Jepson, an Anglican curate, is taking the West Mercia chief constable to court, arguing that he should have prosecuted the doctor who performed a late abortion on a foetus with a cleft lip in 2001.

The 1967 Abortion Act, amended by the 1990 Human Fertilisation and Embryology Act, makes termination of pregnancy legal up to the 24th week. After that, abortions can be carried out only in the case of “severe foetal abnormality”, a term that is not defined in the law and is left to physicians and parents to decide. Cleft lip or palate are malformations of the mouth that occur once in every 700 births. Clefts are not life-threatening and can be surgically corrected in the first year of life.

Jepson argues that cleft lip could not be classed as a “severe abnormality”. She should know, because she was born with a congenital jaw defect. She faced bullying as a child because of her unusual appearance and diction, but surgery between the ages of 17 and 19 solved her difficulties.

To put this case into perspective, some statistics may be helpful. In 2002 there were 175,600 abortions. A large majority were performed under 13 weeks’ gestation. These were cases where women did not want to be pregnant, for social reasons; 1,800 were carried out under the “handicap” clause.

About one-third of these related to Down’s syndrome or other chromosomal abnormalities. Only one abortion was on the grounds of cleft lip or palate, and this was before 24 weeks. A small number — 110 — were terminated after the 24th week of pregnancy because of “severe foetal abnormality”. The majority were cases of congenital abnormalities of brain or heart, lethal chromosomal abnormalities, and other rare conditions.

I support a woman’s right to choose, and I support the right of couples to be given balanced information about a pregnancy, and to choose to terminate pregnancies affected by serious impairments. I do not believe that it is ethical to end a pregnancy, even before the 24th week, on the grounds of trivial abnormalities such as cleft lip.

We live in an age where there is decreasing toleration of disability and abnormality. Modern medicine has promised us perfect babies. Adults and even children now resort to cosmetic surgery, to orthodontics, to lifestyle drugs, to achieve the glossy, happy, buffed and perfect physique and personality that is our inalienable right as consumers.

I have achondroplasia, otherwise known as restricted growth. I inherited it from my father, a successful doctor. I have passed it on to my children, two bright teenagers. We decided that the condition was no bar to happiness, and refused to contemplate termination of pregnancy — to the doctors’ dismay.

In Australia a few years ago a pregnancy was terminated in the 32nd week on the grounds of achondroplasia, because the prospective mother could not cope with the idea of having a dwarf baby. Shockingly, a large proportion of obstetricians believed that aborting a viable pregnancy at that stage was acceptable.

For prospective parents to terminate pregnancy because of minor abnormalities is rare. But it happens and psychological evidence shows how difficult the decision is. Ending a wanted pregnancy as a result of abnormalities detected by ultrasound or amniocentesis is associated with major guilt and distress.

Modern western societies believe knowledge is power, and that choice is an inalienable right, but many couples feel that information from antenatal testing is a burden.

I believe prospective parents have a right to antenatal screening. But I worry that parents do not receive full and balanced information about what disability is like. I fear that it is increasingly difficult for pregnant women to refuse tests, or to choose to continue with their affected pregnancies: to step off the NHS screening conveyor belt. The anthropologist Rayna Rapp writes that these technologies “make every woman into a bioethicist” — men too, I say.

Making abortion illegal after the 24th week, except in cases of ill-defined “severe foetal abnormality” sounds like discrimination against disabled foetuses. Third trimester terminations involve foeticide — injection of potassium chloride into the heart, followed by induced labour.

There are some sad and rare cases where this procedure may be justified. To force parents to take pregnancy to term where a severely impaired baby will certainly die days, weeks or months later seems inhumane.

For prospective parents to demand the right to end their pregnancies on the grounds of minor disabilities in the third trimester is not common. For doctors to accede is even rarer. But abuses undoubtedly occur, and stronger regulation is needed.

The broader lesson of this case is that we should be more accepting of imperfection. Genetics teaches us that every one of us has 100 mutations. Of course parents want healthy and happy children. But there is a limit to what medicine should do to achieve this.

Tom Shakespeare is a social scientist and bioethicist at the University of Newcastle