Scientists have found two genetic regions that affect a baby's size at birth and say one of them is also linked with developing diabetes in later life.
The finding, published in the journal Nature Genetics, is the first firm evidence of a genetic link between low birth weight and diabetes and helps explain why small babies have higher rates of diabetes when they grow up, the scientists said.
Type 2 diabetes—often called adult-onset diabetes—is a common disease that interferes with the body's ability to properly use sugar and insulin, a substance produced by the pancreas which normally lowers blood sugar after eating.
The condition is reaching epidemic levels, with an estimated 180 million people now suffering from diabetes around the world.
Scientists knew lighter babies were more at risk of having type 2 diabetes as adults, but it was not clear why.
It is widely thought that a pregnant mother's eating habits can influence both the growth of her baby and its later risk of disease, a process known as "programming." But Tuesday's research confirms that genes are also important.
"It is now important for us to establish how much of the association is due to our genes and how much is due to the environment, because this will inform how we target efforts to prevent the disease," said Rachel Freathy of the Peninsula Medical School in southern England, who worked on the study.
The research team, which included scientists from Britain, the Netherlands, Finland and the United States, analyzed more than 38,000 Europeans from 19 previous studies of pregnancy and birth.
They found that two genetic variants were strongly linked with birth weight. One of the variants, in a gene called ADCY5, had also been linked with susceptibility to type 2 diabetes.
People who inherit two "risk copies" of this variant are at a 25 percent higher risk of diabetes in adulthood than those who inherit two "non-risk copies," the scientists explained, and this study shows they also weigh less at birth.
Mark McCarthy from Britain's Oxford University, who led the study, said the findings on birth weight were unexpected and showed the combined effect of the two gene regions was "quite substantial."
Nine percent of Europeans inherit two copies of a genetic variant in each region and are, on average, 113g (4 ounces) lighter at birth than the 24 percent who inherit one or no copy.
The effect is equivalent to the reduced birth weight caused by a mother smoking four to five cigarettes a day in pregnancy, McCarthy wrote in the study.
"It was a surprise to see such strong genetic effects for a characteristic such as birth weight, which is subject to powerful influences from so many environmental factors," he said.