Scientists have been able to "unpick" the effects of the genetics of a mother and her baby on the child's birth weight for the first time, researchers say.
Research led by the universities of Exeter, Oxford, Cambridge and Queensland, in Australia, identified 190 links between genetic code and birth weight - two-thirds of which had not been identified before.
The work, the result of a large-scale international collaboration, looked at genetic information from 230,069 mothers with the birth weight of one child each.
Genetic information and birth weights from 321,223 people across the UK Biobank and Early Growth Genetics Consortium cohorts were also examined.
Statistical methods were then used to separate the effects of the mothers' and babies' genes on the weight of newborn babies.
The study, published in Nature Genetics, concluded that the direct effects of a baby's genes - half inherited from their mother and half from their father - made a substantial contribution to birth weight.
Around 25% of the genetic effects identified were from the mother's genes that were not passed to the child but affected growth by influencing factors in the baby's environment during pregnancy, such as the amount of glucose available.
Dr Rachel Freathy, from University of Exeter Medical School, said: "This is the first time we've really been able to unpick the effects of both mother and baby's genes on baby weight, which is an important health indicator.
"It's particularly useful to know about the maternal genetic influences on the environment in the womb because these give us clues as to which factors are causal.
"Better understanding of the causes may mean we can help ensure babies are born at healthy weights."
Babies who are born very large or small have a lower chance of survival and a higher risk of metabolic diseases in later life.
Those who are particularly small at birth have a higher risk of birth complications and tend to be more prone to high blood pressure as adults compared to average weight babies.
Dr Nicole Warrington, from the University of Queensland, said: "The methods we have developed to disentangle the mother and baby's genetic effects have a real potential not only to tell us about the effects of the womb environment on a baby's growth but also about the possible effects of that environment on later-life outcomes.
"For example, smaller babies are more likely to have higher blood pressure in adulthood. Our work shows that this is due to genetic effects; we found no evidence that exposure to the womb environment can raise your blood pressure in later life."
The study found some parts of the genetic code can be linked to birth weight both directly from the child and indirectly from the mother.
A number of these worked together, with the mother and baby effects pushing birth weight in the same direction, while others had opposing effects.
For example, some of the genetic effects that raise the mother's glucose levels work to make the baby bigger because the baby produces more insulin in response, which makes it grow.
But when the same variations in the genetic code are inherited by the child, they restrict the amount of insulin the baby can produce - therefore limiting its growth.
More than 200 researchers from 20 countries who are members of the Early Growth Genetics Consortium were involved in the research.
Dr Robin Beaumont, at the University of Exeter Medical School, said: "This study highlights the value of large-scale international research collaborations.
"It's really satisfying to bring together a wide range of experts to analyse large-scale datasets to advance understanding in key areas of human health."
The major sources of funding for UK researchers in the study were the Wellcome Trust, the Royal Society, the Medical Research Council, the National Institute for Health Research and the European Union.
Maternal And Fetal Genetic Effects On Birth Weight And Their Relevance To Cardio-metabolic Risk Factors is published in Nature Genetics on May 1.