Professor Terry Dwyer, executive director of the George Institute for Global Health, said a "significant amount of evidence" had been compiled which "effectively shows that childhood cancer incidence rises with increasing birth weight".
The International Childhood Cancer Consortium looked at babies and cancer incidence in 380,000 live births, from studies going back more than 50 years.
Professor Dwyer said after adjusting for gestational age and sex, for every kilogram increase in birth weight, there was an increased risk of 26 per cent for all cancers.
"In older children, diagnosed after three, cancers other than leukaemia are particularly related to birth weight," he said.
But interestingly, the mother's weight at birth was not a factor.
"There were no significant interactions with maternal pre-pregnancy weight or pregnancy weight gain," he said.
In Australia, women tend to be older and often heavier when giving birth than in previous decades.
Fast facts on childhood cancer:
- One in 500 Australian children will develop a cancer before 15 years of age — that is 600 Australian children diagnosed every year.
- Childhood cancer is the single greatest cause of death from disease in Australian children, with three children losing their lives to cancer every week.
- Children with cancer are currently treated with adult cancer drugs, because that is all that is available.
The study, published in Paediatric and Perinatal Epidemiology, is the first research to collect data before children develop cancer.
Experts said it was an important finding because very little was known about what causes childhood tumours.
"Now that we know there is a correlation between birth weight and childhood cancer, we can begin to investigate why there is a correlation," Professor Dwyer said.
"It's an important piece of evidence that begins to help fill in the puzzle about childhood cancer."
Professor Dwyer was one of the scientists who uncovered the link between babies sleeping on their stomachs and an increased risk of Sudden Infant Death Syndrome (SIDS).