A gene variant passed down from the mother has been linked to heavier newborns, according to scientists.
The gene, PHLDA2, normally acts as a growth suppressor to limit their size, but the variant, named RS1, can add 93 grams to the baby's birth weight. If inherited from the maternal grandmother via the mother, the weight increase can reach 155 grams.
The team, from the Institute of Child Health in London, looked at over 9,000 DNA samples from three separate groups of mothers and babies. They found the RS1 variant of PHLDA2 in 13 percent, and a more common variant, RS2, in 87 percent of samples.
When the researchers incorporated the RS1 variation into the DNA of human cells in the lab, the PHLDA2 gene, which usually limits birth weight, became less active.
They believe that this is the reason those with the RS1 variation had higher birth weights; a conclusion backed up by previous studies. Mice without the PHLDA2 gene showed placental overgrowth and those over-producing the gene had smaller placentas and a 13 percent reduction in fetal weight.
'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 Professor Moore.
Newborns inherit both maternal and paternal copies of the PHLDA2 gene. However, only the maternal copy is active - the copy inherited from the father is silenced, in a process known as imprinting. It is thought that this happens to conserve resources for the mother and her future offspring.
Birth weight can have implications for lifelong health. Low birth weight, such as that caused by maternal smoking, increases the risk of perinatal mortality as well as the development of adulthood diseases such as type 2 diabetes and obesity. While the PHLDA2 variant increases rather than decreases birth weight, the size of the weight difference is comparable to that caused by maternal smoking.
Speaking to the BBC, Dr Caroline Relton of Newcastle University said: 'Although this study looks only at birth weight as an outcome, it is possible that this genetic variant may have longer-term health consequences'.
The research is published in the American Journal of Human Genetics.