As many as one in 20 of us was at one point one of a pair of twins, research suggests.

Doctors believe that by studying these vanishing twins we can understand the causes of infant birth defects such as cerebral palsy and heart defects.

However, in many cases their existence is not recorded.

New Scientist magazine reports that Profesor Peter Pharoah, an expert in public health at Liverpool University, has conducted a study that shows that if a child has a twin that dies, they are 10 times more likely to develop cerebral palsy.

The normal rate of cerebral palsy is two per 1,000 births.

Re-absorbed

Often when a twin dies its tissues are partially re-absorbed, and form a dry paper-like mass attached to the placenta called a foetus papyraceus.

Professor Pharoah has learned from birth statistics that if a child has a twin foetus papyraceus, his or her chance of having cerebral palsy leaps to 1 in 10.

But of 18 cases recorded in the obstetric records for the Mersey region, a third were registered with the Office of National Statistics as singleton births.

Sometimes the condition is not picked up, and other occasions doctors do not register it to save the feelings of parents.

However, Profesor Pharoah argues that consistent reporting of the foetus papyraceus is crucial, both for medical research and monitoring of the other twin.

He said: "It might be very important for the surviving infant."

Professor Pharoah suspects that the death of a twin may be linked to a wide range of birth defects, from absent kidneys to heart malformations.

Key

The key to the connection lies in the placenta and the chorion, the outer protective membrane that encloses the developing baby in its fluid-filled amniotic sac.

Identical twins come from a single fertilised egg that splits to give two embryos. Often, the twins share a chorion and a placenta. Sometimes they even cuddle together in the same amniotic sac.

The shared placenta of such twins makes them vulnerable to problems, as the placental blood vessels from one twin fuse with those of the other.

This can result in one twin getting far more blood than the other.

Even non-identical twins can share a blood supply. Sometimes in singleton pregnancies, cells from a vanished, non-identical twin turn up in tissue samples. This suggests that the twins once shared the placenta.

Professor Pharoah believes that whenever twins share a blood supply, that blood can shunt to and fro, resulting in the death of one twin and starving the other of oxygen, causing brain damage.

He said: "You are getting a whole gamut of damage.

"I think that a lot of mental retardation is due to the same thing."

Damage

He believes that if the damage happens early in development, when organs such as the heart or kidney are forming, similar shunting could damage these organs as well.

It is estimated that of the 133 million people born in the world last year, at least 7 million should have had a twin.

About three quarters of all conceptions never make it to full term, most being lost in the first few weeks of pregnancy.

Ironically, the chances of loss have increased with the advent of fertility treatments, which can result in a woman carrying several foetuses.

One theory is that the human uterus is simply not adapted for multiple pregnancy.