Researchers found some babies have similar build up of fat around their abdomen as adults in their 50s.
The study of babies at Chelsea and Westminster Hospital, west London, used magnetic resonance imaging (MRI) scans to investigate links between obesity in children and their mothers.
It reportedly found evidence that being overweight or obese in pregnancy could result in potentially harmful changes to a baby's fat levels while still in the womb.
The study, led by Prof Neena Modi, one of Britain's best experts on high-risk health problems in newborns, found nearly a third of children had more fat than expected.
Of the 105 babies – 54 boys and 51 girls – a total of 31 babies had more adipose, or fat, tissue around their abdomen than normal.
Experts said the study was the first direct link that proved the weight of a mother-to-be was passed on to her child and showed that overweight mothers gave birth to fat babies.
"I was very surprised to be able to detect such a clear continuum of effect of maternal BMI (body mass index) on the baby," said Prof Modi, head of neonatal medicine at Imperial College London.
"This is a very important finding indeed, opening the door to a new understanding of how a mother's metabolism affects her baby."
Newborn babies usually have about 700g of adipose tissue, but for each unit increase in maternal BMI, this increased by approximately 7g with a huge build-up in fat in the babies' livers.
Prof Modi, who is also a consultant neonatologist at Chelsea and Westminster Hospital, told the Daily Mail: 'This shows how sensitive the baby is to the environment experienced within the womb and how lifelong effects may be initiated before birth.'
The World Health Organisation classes a BMI between 18.5 and 25 as normal weight, between 25 and 30 as overweight and over 30 as obese.
Are you shocked by this research? Are you pregnant and trying to eat healthily?