A landmark new medical study has confirmed Australia's ageing first-time mums have sent the nation's rates of caesarean section births soaring.

Increasingly, overweight and obese new mothers are also emerging as a factor driving the massive growth in c-sections.

The findings have prompted researchers to call for a rethink in attitudes to first births and fertility.

The research, to be published in the Australia and New Zealand Journal of Obstetrics and Gynaecology, looked at the first births of more than 117,000 mothers in South Australia over a 20-year period.

The mother's age was found to be a factor in three-quarters of caesarean interventions, casting doubt on the theory that decision-making by doctors was the main reason for the explosion in caesarean rates.

The rate of c-section interventions in Australian births rose from 18 per cent of the total in 1991 to more than 31 per cent in 2010, an increase of more than 75 per cent.

The reasons behind the increases have been unclear until now and the researchers from the Australian National University, Flinders University and other SA institutions noted that efforts to reduce the incidence of caesareans have largely failed.

The project leader, ANU Associate Professor Steve Robson, said the data from the study provided the clearest evidence yet that age mattered in the caesarean section debate.

"Nobody has been able to pin down what are the really important things and what sort of contribution they make," Dr Robson said. "Our analysis suggests that probably the biggest single factor that seems to be driving this, and it might well play a part in three-quarters of the increase, is the fact that women are leaving the decision to have their first baby until later in life.

"So now we've put a number to it, it surprised us just how big a part that women delaying child-bearing seems to play."

But with medical opinion united that vaginal births are better than c-sections, Dr Robson said the ageing profile of first-time mothers was a tough problem to tackle.

"That's a really hard thing to do anything about, couples leaving childbirth until later in life," he said.

"Maybe it's time for society to look at ways to make it easier for women to have their babies earlier in life, I think a lot of complications could be avoided that way.

"We know it's easier to get pregnant, it's easier to stay pregnant, it's easier to have baby, the younger you are. There's so much talk about things like paid parental leave and so on but I think that as a community we need to have another look at how important it is to make it easier for couples to have children when they're younger."

Dr Robson said that the mother's weight was also emerging as a factor in c-sections but the research task was hampered by a lack of data.

"We know it's a very big factor but the problem is we only started recording the mother's weight in the past couple of years. No doubt it's having an effect as well but it's hard to say what that effect is."