Scientists from University College London have found that a gene variation that is inherited from the maternal grandmother can add up to 155g (5.5oz) to birthweight.
The research, led by Professor Gudrun Moore and published in the American Journal of Human Genetics, focussed on a gene called PHLDA2, analysing DNA samples from nearly 9,500 mothers and their babies. The gene is usually used to suppress growth, but a variant called RS1 appeared to change the way the gene functioned, leading to increased birthweights.
Professor Moore told the BBC: "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 93g bigger on average, or even 155g bigger on average, if inherited successively from the mother's mother."
13 per cent of the samples analysed possessed the RS1 variation, with the rest possessing the RS2 variation. The more common gene variation, which is only found in humans, is a proactive gene that may have evolved to reduce the risk of death at birth. This evolutionary theory may also explain why the dad has relatively little genetic engagement at birth, as the male has been less prone to risk over time.