Australian babies are becoming increasingly big bundles of joy, and health experts warn it's not always something to be happy about.

A positive decline in smoking during pregnancy - resulting in less low birth weight babies - has occurred since 1990, according to a study that took in almost 1.3 million births.

Mothers with gestational diabetes, resulting in heavier babies, also rose and these two factors contributed to a rising rate of newborns weighing in at more than four kilograms.

The study, published in the Medical Journal of Australia, looked at babies born in NSW in the 16 years to 2005.

Dr Ruth Hadfield, who conducted the study with her University of Sydney colleagues, said heavier babies had increased health risks, contrary to the popular notion that big babies were healthy babies.

"For example, there is evidence of a relationship between high birth weight and the increased future risk of asthma, type one diabetes and a number of cancers, including infant and childhood leukaemia, and breast, prostate and colon cancer," said Dr Hadfield, who is postdoctoral research fellow at the University of Sydney at Royal North Shore Hospital.

"As well as the potential for lifelong health consequences, higher birth weights may also result in injury to the infant and the mother at the time of delivery."

During the 16-year study period, newborn boys weighing in at more than 4kg increased 10.5 per cent.

For newborn girls, the figure was 15.2 per cent.

In 2005, one in six newborn boys and one in 10 newborn girls tipped the scales at over 4kg.

Another contributing factor was the fact more women were waiting later in life to have children, but Dr Hadfield also said not all of the increase could be accounted for.

"Although decreasing smoking, increasing maternal age and increasing gestational diabetes account for a portion of the increase, a larger portion of the increase remains unexplained by our data," she said.

Dr Hadfield said further studies should look at whether there was a trend of women putting on more weight during pregnancy.

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