Australia is experiencing an "epidemic" of large babies due to the high rates of obesity, according to a new study.

Analysis of more than 3,000 pregnant Sydney women shows the number of babies born between 2010-2012 classified as Large for Gestational Age (LGA) - weighing more than 4000 grams (4 kilos) - was significantly higher than expected by the international standard.

The study published in The Royal Australian and New Zealand Journal of Obstetricians and Gynaecologists found 16.5 per cent of the infants born in this time had birthweights greater than the 90th percentile using the international INTERGROWTH21st standard.

Higher maternal body mass index (BMI) and pre-existing diabetes increased the chances of having a larger baby.

The number of infants classified as Small for Gestational Age (SGA) was also significantly lower.

"This likely reflects the prevalence of obesity in an Australian setting where one in two women who become pregnant are overweight or obese," the authors wrote.

Over-sized babies can cause complications for both the mother and baby, warn the researchers.

"Maternal adverse outcomes include emergency caesarean section, birth trauma, post-partum haemorrhage and maternal diabetes," the authors at John Hunter Hospital wrote.

Along with the immediate complications for the baby, studies have also linked birth weight to long term health outcomes, they say.

"Longitudinal data have convincingly demonstrated increased risks of longer term complications, including childhood obesity, type 2 diabetes and metabolic syndrome in adolescence and adulthood," the authors wrote.

The researchers say greater interventions to reduce the risk factors before and during pregnancy are needed "if we are to make any significant impacts on the obesity epidemic for the next generation."