New Zealand and Australian pregnant women who weigh more than 140kg are at greater risk of complications including gestational diabetes and caesarean section deliveries, new figures show.

Almost 750 pregnant women in both countries are extremely morbidly obese, according to fresh data from the Australasian Maternity Outcomes Surveillance System (AMOSS).

These women have a body mass index (BMI) greater than 50 or weigh more than 140kg kilograms. A healthy adult BMI is between 20 and 25.

The figures, to be presented on Tuesday at the Australian and New Zealand College of Anaesthetists (ANZCA) conference in Perth, showed extremely morbidly obese women were twice as likely to require a caesarean delivery than other pregnant women.

A more detailed study of 370 Australian women in the survey showed 38 per cent had their labour induced, compared with 21 per cent in the general population, and more than half (52 per cent) had a caesarean delivery, compared with 32 per cent of other pregnant women.

There were also higher rates of gestational hypertension (13 per cent), pre-eclampsia (nine per cent) and gestational diabetes (15 per cent) in extremely morbidly obese pregnant women.

Although there were no deaths among the women surveyed, AMOSS Investigator Nolan McDonnell from Perth's King Edward Memorial Hospital for Women said these women were at greater risk of death than other pregnant women.

"Obesity is known to be a significant risk factor for maternal mortality," Dr McDonell said.

Surgery was far more complicated in this group of pregnant women, who were more likely to suffer infections and require intensive care, he said.

Dr McDonell said obese women should be encouraged to lose weight before becoming pregnant and limit their weight gain during pregnancy to reduce the risk of complications.