London: Scientists have discovered a genetic variation which they say could contribute up to 155 gram to a child's birthweight.

If the particular variant is passed down from the mother, it can add up to 93 grams to the baby's birthweight and if passed down from the maternal grandmother, it could add up to 155 grams to the baby weight, according to researchers from the University College London.

The gene examined in the study was believed to act as a growth suppressor, reducing birthweight, the BBC reported.

For their study, published in the American Journal of Human Genetics, the team led by Prof Gudrun Moore looked at a gene called PHLDA2 in nearly 9,500 DNA samples taken from mothers and their babies, collected in three separate studies.

They found a gene variant called RS1 appeared to change the way in which the gene functioned, leading to higher birthweights.

"The gene is already known to have a profound effect on birthweight by acting as a growth suppressor. We have found a genetic variant of PHLDA2 that when inherited from the mother, causes the baby to be 93gm bigger on average, or even 155gm bigger on average, if inherited successively from the mother's mother," Prof Moore said.

According to the researchers, the RS1 variation was found in around 13 per cent of the individuals studied, with 87 per cent possessing the RS2 variation.

"We suggest that the more common RS2 gene variation which is only found in humans has evolved to produce a smaller baby as a protective effect to enhance the mother's survival during childbirth," said Prof Moore.

"Dad's lack of involvement in evolutionary terms may stem from his own survival not being at stake and he can continue to reproduce with other females." The PHLDA2 gene is unusual in that as only the copy inherited from the mother is active, while the copy inheritedfrom the father is "silenced". This silencing of the paternal gene results from molecular processes around the DNA known as epigenetics.

Scientists do not know why, but have speculated that it is to ensure birthweight is reduced to ensure the mother survives childbirth.

Dr Caroline Relton of Newcastle University said: "Though this study looks only at birthweight as an outcome, it is possible that this genetic variant may have longer-term health consequences.

"Indeed the long-term health consequences associated with extremes of birthweight might be due in part to this and other contributory genetic factors."