Several medical treatments pose the risk of birth abnormalities. Tetracycline antibiotics affect hard tissue formation during the foetal period, affecting the long bones and teeth. Some anticonvulsant medicines used for epileptics may also cause problems. Anyone on long-term medication who is planning to become pregnant, or who has become pregnant, should discuss their situation with their doctor.

Moderate alcohol use in pregnancy is a particularly tricky problem. Many women who abuse alcohol may also have unhealthy lifestyles - inadequate diet, smoking and lack of exercise, and this confuses the issue in establishing the exact role of alcohol. However, excessive use of alcohol is certainly damaging to the baby. Occasional use has not yet been clearly shown to pose a risk. Current UK Department of Health guidance advocates no more than one or two drinks once or twice a week. However, the decision on light drinking must be a personal one. It is virtually impossible to guarantee anything as completely 'safe' in pregnancy.

Smoking is bad for the development of the baby in a number of ways. Pregnant women should not smoke under any circumstances.

Lack of something may be as damaging as the presence of something. Extensive trials have confirmed that taking folic acid before pregnancy can reduce the number of spinal cord defects such as spina bifida.

In general, if potential and expectant mothers eat a healthy and varied diet including fresh fruit and vegetables, and fish, this may well reduce the likelihood of birth defects.

Vitamin supplements are also potentially useful, but care must be taken to avoid too much vitamin A.

There is a clear association between poverty and birth defects, and this may be related to diet as well as education.

Biological factors, including illnesses
The rubella virus, which causes German measles, poses a serious hazard to the developing baby. It can cross the placenta and attack the baby to cause a range of defects. The proportion of mothers infected with the rubella virus during the first 12 weeks of pregnancy who have an abnormal child can reach 20 per cent. This is why it is so important to make sure that everyone is vaccinated against rubella.

Other viruses, such as cytomegalovirus and herpes simplex, can also cause problems. Bacteria such as salmonella, present in undercooked meat (especially chicken), and listeria (found in some cheeses), can cause problems for the continuation of the pregnancy (rather than birth defects).

Physical factors (including radiation)
X-rays and radiation given as cancer therapy can affect the baby. Your medical carers will be very cautious about giving you an abdominal X-ray if you are pregnant or potentially pregnant (this means almost every woman of child-bearing age!). With proper shielding, X-rays can be given to other parts of the body during pregnancy, in the case of emergencies.

When is my baby most vulnerable?
The time at which the developing baby is exposed to a possibly damaging environmental factor is more important in determining the nature of the likely abnormality than the nature of the factor itself. In fact, if babies are exposed to the same damaging factor at different times in their development, the nature of the damage is likely to be different. Conversely, if babies are exposed to different damaging factors at the same stage in development, the damage caused may well be similar. For instance, exposure to the rubella virus at three weeks after fertilisation may cause heart defects, and at six weeks after fertilisation, it may cause deafness. Exposure to, say, radiation at three weeks may cause similar damage to rubella. Examples of a specific factor always causing a single specific defect are less common than is generally thought.

The developing baby is particularly vulnerable to environmental factors during a five-week period starting about three weeks after fertilisation, and lasting till about the end of the eighth week after fertilisation. (Note that pregnancy is usually timed from the last menstrual period, which is generally about a fortnight before fertilisation. 'Three weeks after fertilisation' therefore corresponds to the 'fifth week of pregnancy' and 'eight weeks after fertilisation' to the '10th week of pregnancy'.

This five-week interval is often called the 'sensitive period' as it is crucial to normal development. Even within this period, the body organs form at different times. Sometimes it is possible to estimate retrospectively from the nature of the defect when the baby might have been exposed to a possible environmental factor. The mother's medical history can then be examined to see what she was exposed to over this period.

After the sensitive period, major physical malformations become less likely. However, the developing baby is still at risk from environmental hazards. During the remainder of development, tissues grow and mature, and some, such as the teeth and bones, can be quite vulnerable. Most significant of all is the development of the nervous system, particularly the brain. Here, many complex connections are taking place throughout development, and these also may be susceptible to environmental effects.

How can birth defects be detected in advance?
Diagnosis of birth defects has been revolutionised by the widespread use of ultrasound. Ultrasound scanning seems to be very safe in normal use. Properly used, this can currently detect about 50 per cent of abnormalities before birth. Spina bifida can be diagnosed very accurately, as can major external abnormalities such as defects of the wall of the abdomen

On rare occasions, a developing baby that is actually normal may be suspected of having an abnormality.

Blood tests for expectant mothers are also valuable. The 'triple test' is widely used. In this, levels of alpha-fetoprotein, human chorionic gonadotrophin and oestriol in the mother's blood are measured. Various abnormalities are associated with alterations in their levels. However, the levels can also vary between individual women, and such tests can only provide estimates of risks, rather than proof of the presence or absence of an abnormality. Thus, mothers are often informed of the size of the possible risk, and advised to seek further tests, as this screening may still miss some problems.

A common further test is amniocentesis, when a sample of the fluid surrounding the baby is removed, and the cells in the fluid are checked. This procedure poses a small risk of inducing a termination of pregnancy.

What can be done about birth defects when they are detected?
For many serious defects detected before birth, termination of pregnancy is presently the only option other than non-intervention. This is a difficult ethical choice for many people. However, it has had a significant impact. In Glasgow and Dublin, detection of spinal cord defects during pregnancy are similar. Yet more than twice as many children with spinal cord defects are born in Dublin than in Glasgow, because voluntary termination of pregnancy is illegal in the Irish Republic.

However, advance detection of abnormalities can also lead to better preparation at the time of birth. The parents may be better prepared emotionally - and better informed. Surgical teams can also be ready to carry out emergency procedures if the condition is immediately life threatening.

Attempts have been made to correct abnormalities by surgery on the babies before birth. While there have been some notable successes, there have also been failures. Opening the womb alone can cause serious difficulties for the mother, quite apart from the effect of the operation on the baby. Technological advances should make foetal surgery more successful, but it is likely to be reserved for serious conditions for some time.

Finally, people with birth disabilities often meet with a lack of understanding and compassion, despite their brave efforts to lead rewarding lives. A greater degree of understanding and tolerance by society in general would be enormously beneficial.

Who is to blame?
Having a child with a serious abnormality can be a shattering event for many couples. They may suffer from irrational feelings of guilt, believing that they are to blame for the problem. Equally, they may deflect blame by blaming others, eg doctors. The question of responsibility may become entangled with legal liability. However, parents are not to blame in any way for the great majority of birth defects. Indeed, no one may be to blame for what may represent a tragic accident

How can we reduce the numbers of birth defects?

Medical advances can be expected to increase the range of options open to parents when a problem is detected. The number of birth defects is falling in Europe, and this seems likely to continue as part of generally increasing health levels.

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