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Birth defects

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Birth defects

Written by Dr John McLachlan, senior lecturer in medical science

Is there something wrong with my baby?

One of the most common worries for expectant parents is that something will be wrong with their baby. However, most babies are born without problems, while most of those who are can have treatment that will make them much better, or even allow them to lead a normal life. Understanding the nature and causes of birth defects can help reduce the mystery, fear and guilt often associated with them.

How common are birth defects?

An international monitoring programmeshows that, across Europe, just over 2 per cent of babies are discovered at birth to have a defect that will affect their ability to survive or function normally. Some defects are often not discovered until later (eg deafness and problems in walking), and these are not included in this figure.

There is considerable regional variation in the rate of defects seen at birth. Glasgow and Dublin have traditionally been black spots, with rates over 3 per cent, but these figures are falling. The number of babies actually born with defects is also markedly affected by the availability of prenatal screening, and the availability of voluntary termination of pregnancy in the case of severe abnormalities.

What form do defects take?

Defects of limbs, heart and spinal cord represent about half of all abnormalities.

The most common kinds of defect are those affecting the limbs (arms and legs). These include missing or extra fingers or toes, deficiencies in limb length, and abnormalities in positioning, such as club foot.

Heart abnormalities represent the next most common category of defect. Common heart defects include 'holes in the heart' where blood can pass from one side of the heart to the other. Again, these may not all be detected at birth.

The third most common kind of defect affects the spinal cord, such as spina bifida.

Other defects commonly observed include those affecting the face (such as cleft lip and palate), problems with the development of the intestines and stomach, and problems affecting the sexual organs.

Major chromosomal problems such as Down's syndrome (Trisomy 21) are found in about 0.15 per cent of births (about three babies in every 2000).

What causes birth defects?

Genetics In ancient times, abnormalities were seen as warnings or punishments from the gods. Even quite recently, it was believed that specific events during the pregnancy - such as being frightened by a mouse - would lead to specific defects in the baby, eg a mouse shaped birth mark. Such folk beliefs still persist in places. However, most defects that can be traced to a single cause have a genetic origin. This does not necessarily mean that the parents suffer from the same defect. It may be that they are carriers of the condition, or that the genetic problem occurred for the first time in the cells that gave rise to the baby. Known genetic causes account for about 25 per cent of all abnormalities.

Environment

Environmental causes (including exposure to drugs, radiation and illnesses) can be identified in about 10 per cent of birth defects. The risk of exposure to such environmental factors often causes great concern, although genetic causes are more common.

Other factors

The exact causes of the remaining 65 per cent of defects are currently unclear. Many of these defects arise from an interplay of genetic and environmental factors. For instance, some babies may have a genetic susceptibility to certain environmental factors. If exposed to such factors during development, they may develop an abnormality. If they are not exposed to the factor, they will be perfectly normal. Another baby exposed to the same environmental factor, but who doesn't have the genetic susceptibility, may be entirely normal. This can make it difficult to pin down the exact cause of a defect.

The Human Genome Project has worked out the normal sequence of all genes found in people. In the future, this will be invaluable in teasing out the relative roles of genetics and the environment in causing birth defects.

What are the main environmental factors in causing birth defects?

There are three main kinds of factor that can cause defects: chemical, biological and physical.

Chemical factors, including drugs Thalidomide is perhaps the most notorious example of a drug that caused defects. It was brought on to the market after inadequate testing and specifically prescribed for pregnant women. Following this, much stricter rules about drug testing were introduced, and international monitoring programmes were put in place. Doctors are also much more cautious about prescribing drugs to women of child-bearing age, so it is unlikely that a disaster on this scale will occur again in the UK.

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 epilepsy 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.

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