TERATOGENESIS

Certain environmental and genetic factors can cause fetal deformity, or teratogenesis. Medical research is concerned with preventing such malformations and detecting them early in the pregnancy.

HOW DEFORMITIES OCCUR

Most congenital malformations develop during the first 12 weeks of pregnancy when the fetus is growing very rapidly and developing the different limbs and organs. This is when the teratogen would have the most effect. Later on, it is the general growth that is affected. Sometimes a substance that damages a fetus if taken early in the pregnancy may have no effect if taken later on.

It also seems likely that some families inherit a factor which makes them more susceptible to a teraogenic substance.

There are several different ways in which teratogens can affect a fetus's development. Parts of the body such as the heart or an arm may not develop at all, may be smaller than normal, or the wrong shape. The fingers or toes may not divide properly (syndactyly); the intestines may not canalize properly and be blocked from birth; or they may be in an abnormal position (malrotation). Sometimes there is an extra finger or toe (polydactyly).

These and other abnormalities may be very minor and easily corrected, but they may be more serious, particularly if they affect the brain or the heart.

DRUGS AS A CAUSATIVE FACTOR

Drugs play a fairly small part in the cause of congenital malformation, but as nearly 80 per cent of pregnant women take drugs other than iron tablets, it is very important to know which drugs can be harmful.

The way in which drugs damage the fetus's cells is not really understood, so it is difficult to predict which drugs will cause malformations. The same drug may produce different types of abnormalities in different people and although a drug may affect humanism it may not show any effect when tested on other animals. Most of our knowledge about drugs comes from reports of babies being born with malformations after their mother had taken a certain drug. The other problem is that it is sometimes difficult to know whether it is the drug to be taken, that affected the fetus as it developed.

A few drugs are definitely known to be teratogenic. These include thalidomide and most of the drugs used for treating cancer. There is also a group of various drugs which probably affect the fetus and should be avoided if possible. These include alcohol, some steroids, some of the drugs used to treat epilepsy and live vaccines (rubella and smallpox) LSD, barbiturates, a few of the antibiotics and frequent contact with anaesthetic gases are possible teratogenics but there is no definite proof about their effects.

RADIATION AND INFECTIONS

Small doses of radiation, such as those emitted during a chest X-ray, are probably not harmful. However, radio therapy ( a type of medical treatment) and the radiation from nuclear explosions kill or damage the unborn baby, particularly affecting the brain. Unfortunately there is sufficient evidence of the horrific effects of radiation on the fetus from the bombing of Hiroshima and Nagasaki.

Several viruses are known to be teratogenic. T he infection spreads from the bloodstream of the mother across the placenta and into the fetal bloodstream. Rubella (German measles) is the commonest teratogen. The risk to the fetus is greatest during the earliest part of the pregnancy. There is a 50 per cent chance that the fetus will be affected if the mother catches rubella in the first four weeks. The risk falls to 22 per cent up to the 8th week and to 6 to 8 per cent from the 12th to 20th week; after this it is uncommon for there to be any effect. The commonest abnormality is deafness, followed by congenital heart disease, retardation and cataracts.

A rarer teratogenic agent is cytomegal, or salivary gland, virus. Unlike rubella, there is no immunization.

RECOGNIZING MALFORMATIONS

Recognizing that a fetus has been affected by a teratogen and will be malformed can be extremely difficult, as many illnesses and many malformations do not produce any symptoms during pregnancy. Often the affected fetus will die and a miscarriage occur.

In spinal bifid (where the bones and nerves of the spine do not fuse together properly) there is a raised level of a certain protein in the blood and the amniotic fluid (the fluid surrounding the baby in the womb). This can be detected at about 16 weeks of the pregnancy, and in many obstetric units this is a routine blood test given to all mothers.

An ultrasound scanner can also pick up this deformity, and recently other congenital defects of the heart, kidneys and intestines are also being detected as the techniques are improved and the technicians and doctors become more expert at using them.

The best way to reduce the risk of having a malformed baby is to see your doctor as soon as your pregnancy is confirmed and tell him or her of any family history of abnormality and of any worries you may have. Avoid taking drugs whenever possible, have a good balanced diet and take the iron and vitamin tablets your doctor will prescribe. Women with PKU must return to a low phenylalanine diet before becoming pregnant.