Gene battle 'may cause small babies'

Although babies born prematurely are likely to weigh less than those carried to full term, some are born small for no apparent reason.

Researchers from the University of Cambridge, UK, say they have found clues that the root of this problem can be found early in pregnancy.

They have found that some babies have too little of a molecule which appears to play a role in developing the placenta.

The researchers say that the finding could help them monitor women for early signs that they are likely to produce a small baby.

Professor Gordon Smith, who led the research, said: "Further research is needed to identify those women with high risk pregnancies in the very early stages so that babies with the greatest risk of complications can be monitored."

A spokesman for the premature baby charity Bliss said: "To be able to predict if or when a pregnant woman is going to have a premature or low birthweight baby will have a fundamental effect on the baby's treatment and outcome.

"Medical and nursing staff will be able to monitor the mother more closely, as well as anticipate and plan the appropriate care for the baby.

"Additionally, the parents will be able to come to terms with an early or low-birthweight birth."

However, while this finding could eventually be of practical benefit to women and their babies, it, and another Cambridge University finding, may lend credence to a theory that birthweight may be a way for the mother to give her genes the most chance of survival.

Some scientists believe that some human genes are left over from days when scarce resources meant that the average 2.4 children could not be taken for granted, particularly if one large baby drained too much from the mother.

The more babies a woman has, the more chance there is that her genes will pass on to a further generation.

Stressful pregnancy

However, having a baby places immense stresses on the body, and in times of poorer nutrition and health, having a bigger baby might reduce a woman's chance of surviving to give birth to many more.

So, in theory, it would be advantageous to have higher numbers of slightly smaller babies.

Equally, if a man was having babies with a number of different partners, it would be better for him to have as large a baby as possible with each.

This means that the man and woman are in unknowing competition for the survival of their genetic code.

The second Cambridge study looked at the role of a gene called IGF on the development of the placenta - and the size of the baby.

Stunted mouse

This gene comes from the father, and appears to increase the size of the placenta, and thereby boost the flow of nutrients to the foetus.

Mice with a placenta lacking the IGF gene had significantly smaller offspring, the researchers found.

The theory is that if IGF is the father's "weapon" to boost the size of the infant. It is possible that some women have similar genetic tools which help them control the effectiveness of IGF in increasing placental size.

It is possible that a gene restricting the production of PAPP-A could be one of them.

The research is published in the journal Nature.