Medical experts are calling for a national strategy to help women maintain a 'normal' weight before they conceive a baby to avoid serious complications.

A steady increase in the rate of overweight and obese first time mums is to blame for a "substantial" proportion of serious pregnancy and birth complications, according to new Australian research.

Medical experts say helping women loose weight prior to conceiving a baby needs to be the focus of a national approach to combat the public health issue

"We found that a substantial proportion of the burden of adverse perinatal outcomes for Australian women is linked to maternal overweight and obesity, and that this proportion has steadily increased over the past 25 years," wrote the authors of a study published in the latest edition of the Medical Journal of Australia.

Researchers, led by gynaecologist Associate Professor Kirsten Black and Ms Kate Cheney, analysed the data for more than 42,000 first-time mothers who gave birth to a single child at the Royal Prince Alfred Hospital in Sydney between January 1990 and December 2014.

They found that the prevalence of mothers who were overweight increased from 12.7 per cent in 1990-1994 to 16.4 per cent in 2010-2014.

The prevalence of obesity rose from 4.8 per cent to 7.3 per cent over the same period, while the proportion of women with a 'normal' weight range fell from 73.5 per cent to 68.2 per cent.

The researchers then estimated the percentage of adverse perinatal outcomes that were attributed by exposure to being overweight and obese.

For almost one in four (23.8 per cent) pregnant women who had pre-eclampsia between 2010-14, the condition was attributable to carrying too much weight.

Being overweight or obese was associated with 17 per cent of gestational diabetes cases and 23.4 per cent of fetal macrosomia (larger than average baby).

However many of these complications could have be averted through weight loss, the authors said.

The study found if the overweight and obese women were to have moved down one BMI (Body Mass Index) category during 2010-2014, 19 per cent of pre-eclampsia, 14 per cent of gestational diabetes and 8.5 per cent of caesarean deliveries could have been prevented.

BMI is a measurement tool used to determine whether a person is in a healthy weight range for their height.

"Importantly for practice and policy, our results indicate that the frequency of adverse perinatal outcomes could be reduced by shifting the distribution of overweight and obesity among first-time mothers by a single BMI class," the authors wrote.

"Investing in obesity prevention strategies that target women prior to their becoming pregnant is likely to provide the greatest benefit."