We just can't get enough of twins

Published on Friday 22 August 2008 20:08

THE birth of twins may cause anxiety for parents, particularly if the babies are premature and of low birthweight.

Indeed, studies on twins born in Europe during the 1930s showed they had a lower IQ and academic achievement than comparable singletons. But in more recent studies, on those born in the 1980s, this difference has vanished.

This good news for twins is probably due to improvements in living conditions and antenatal, postnatal and paediatric care for low-birthweight babies.

Twins are not uncommon; in the UK, one in 65 pregnancies produces them, so about one in 32 of us is a twin.

One-third of these are monozygous – genetically identical, same-sex twins arising from a single egg that splits early in development. The other two-thirds are dizygous – non-identical twins arising from two separate, fertilised eggs and being 50 per cent genetically identical, just like other siblings.

Numbers of monozygotic twins throughout the world are fairly constant, but the incidence of dizygotic twins varies, reportedly highest among the Yoruba people of West Africa, where twins arrive at the rate of approximately one per 22 births.

In many western societies, including Britain, the numbers of dizygotic twin births have risen because of fertility treatments involving drugs, such as Clomid, that induce multiple ovulation, or the implantation of two embryos after in-vitro fertilisation.

To most, twins are cute, but to scientists interested in the genetic origin of disease, they are absolutely fascinating. They are in high demand for medical research, as they provide a unique opportunity for unravelling the age-old nature v nurture controversy; that is, the relative influence of genetic and environmental factors on life events.

Unless separated at birth, twins have the same nurturing during childhood, sharing everything from the womb and birth date to family, home and, usually, school. So, comparing the incidence of a particular condition, such as diabetes or obesity, between 100 per cent genetically identical monozygotic twins and 50 per cent identical dizygotic twins will reveal any genetic predisposition.

Scientists give this predisposition a percentage score called heritability, and have used twins to determine the heritability within the general population of a wide variety of conditions, including asthma, weighing in at 60 per cent heritable, schizophrenia at 80 per cent, and breast cancer at 30 per cent.

In conditions with high heritability, the race is on to find the human susceptibility genes.

The foetal origins hypothesis suggests that babies whose growth is restricted in the womb during the last three months of pregnancy have an increased risk of early death from conditions such as heart disease and diabetes. Twins are ideal for testing this hypothesis, since virtually all of them have their growth in the womb curtailed by their co-twin.

Interestingly, large studies from Scandinavia comparing all causes of death between adult twins and singletons show no differences, except in suicide. This is less common in twins, perhaps because of the strong bond that exists between them.

Fortunately, twins seem to be very willing guinea pigs, happily giving their time and donating blood and DNA for medical research that is unlikely to benefit them directly. Indeed, there are many twin registries around the world set up with the express purpose of aiding research, and there is even a medical journal, entitled Twin Research and Human Genetics, dedicated entirely to the subject.

But there are also many research projects that directly address the welfare of twins. A recent report in the British Medical Journal on twins from Taiwan born in the 1980s indicates that Asian twins are following their European counterparts, with recent improvement in their academic achievements.

Encouragingly, the differences between Taiwanese twins and singletons are slight, with twins being only 2 per cent less likely to attend college. This figure falls between the 1930 and 1980 European twin figures and fits with the upward trend seen in Europe. Thus, the difference can be expected to disappear as antenatal and postnatal care in Asia continue to improve.

&#149 Dorothy H Crawford is a professor of medical microbiology and assistant principal for public understanding of medicine at Edinburgh University.