Women who have an unhealthily wide waistline before pregnancy are more likely to deliver a baby with autism, major new research suggests.

While previous research has found no link between autism and pre-motherhood obesity, every study to date has used body mass index as the primary metric, which does not discriminate between fat and lean mass.

Switching to assess waist circumference, Northwestern University researchers found a clear correlation between obesity in young women and autism in the children they had years later.

The landmark finding, presented today at the Endocrine Society's annual meeting in Chicago, is the latest in a boom of evidence suggesting the rising rate of obesity globally is linked to a spike in autism diagnoses, and it likely has something to do with inflammation.

The paper also adds more weight to the growing campaign to focus on obesity among young women as a critical public health concern, not least to curb the staggeringly high rates of American women dying in childbirth from heart complications.

A Northwestern study found a clear link between obesity in young women and autism in their children, despite previous studies finding no link. Previous studies used BMI as their metric

'Children born to mothers with a waist of 80 centimeters (31.5 inches) or more before pregnancy showed a 65 percent increase in the risk of autism than those born to a mother with a smaller waist,' said lead author Dr Geum Joon Cho, visiting scholar of obstetrics and gynecology at the Feinberg School of Medicine, Northwestern University in Chicago.

'It is assumed there are multiple factors that cause autism, both inherited and environmental,' Cho, who is also associate professor of obstetrics at the Korea University College of Medicine in Seoul, said.

'Of the environmental risk factors, emerging evidence has linked maternal pre-pregnancy obesity to the risk of autism in offspring.

'However, other studies have reported no associations between the two conditions. We wanted to investigate this association further.'

According to Dr Cho, waist circumference is the best way to measure visceral fat - body fat stored inside the abdominal cavity which, as a result, comes into close contact with organs such as the liver, pancreas and intestines.

It is hardly the first time someone has suggested such a change. For years, people have rallied against the 'archaic' BMI approach, which measures height and weight to produce a result (meaning Donald Trump has the same BMI as Tom Brady and Jason Momoa).

Dr Cho's team analyzed data on 36,451 mothers, who each gave birth between 2007 and 2008, and each got screened within a year of their pregnancy.

Many women in that group were classed as obese using the body mass index alone, but just a handful who had an obese waist circumference (more than 31.5 inches).

Next, researchers tracked the women's babies for seven years. They found 265 of them (0.76 percent) had been diagnosed with autism spectrum disorder.

Comparing the two data sets, they found obesity as measured by BMI had no correlation with autism.

However, overwhelmingly, most of the babies diagnosed with autism (65 percent) were born to mothers with an obese waist circumference.

Dr Cho, who was not surprised by the results, said the link is likely down to inflammation, since that is a key factor in both obesity and autism.

'Both intrauterine inflammation and fetal brain inflammation are implicated in the development of autism,' Dr Cho said.

'As obesity increases, circulating immune system proteins called inflammatory cytokines in pregnant women and the inflammation associated with maternal obesity may be related to the development of autism.

'Waist circumference, as a measure of central obesity, is associated with an increase in inflammatory cytokines, which is known to be involved in the development of autism.'

'The findings suggest the need for clinicians to monitor for maternal obesity, based on waist circumference, to minimize the risk of development of autism spectrum disorder in offspring,' Dr Cho said.

'Further studies are needed to evaluate whether altering maternal waist circumference would lessen the risk of the development of autism in offspring.'