HUNDREDS of extremely obese women weighing more than 140 kilograms are giving birth with serious complications in Australia - and doctors fear the trend will continue to worsen.

The Australasian Maternity Outcomes Surveillance System found 370 - or two in 1000 - pregnant women in Australia had extreme morbid obesity in 2010 - double the rate in Britain. Extreme morbid obesity was defined as a body mass index of more than 50 and/or a weight over 140 kilograms. A healthy adult should have a BMI of between 20 and 25.

While tens of thousands of obese Australians are giving birth each year, Dr Nolan McDonnell, an anaesthetist at the King Edward Memorial Hospital for Women in Perth, said the surveillance system had provided the first snapshot of how many pregnant women fell into this extreme category.

He said the research showed a much higher rate of complications, including life-threatening problems that require more intensive care during pregnancy, birth and after birth.

Those with extreme obesity were nearly 10 times more likely than those without to have high blood pressure, three times more likely to get pre-eclampsia and twice as likely to have gestational diabetes. They were also nearly four times more likely to be induced, twice as likely to require a caesarean and nearly five times more likely to end up in intensive care units after birth.

With more Australians becoming obese, Dr McDonnell said he expected numbers of extremely obese pregnant women to increase. ''The data suggests that rates of extreme obesity pregnancy are higher than the United Kingdom and they are increasing,'' he said.

''Based on this research, we encourage women who are obese to get expert assistance if they are planning a pregnancy, to lose weight beforehand, to limit weight gain during their pregnancy, and to ensure they have access to high-quality care to best manage their pregnancy.''

Dr McDonnell said extreme obesity significantly increased the risk of death for pregnant women, with British data showing about half of the 227 women who died during pregnancy or shortly after birth between 2006 and 2008 were obese. Of the 370 extremely obese pregnant women in Australia in 2010, none died before, during or after birth, but two were admitted to intensive care units after birth and five lost more than a litre of blood during birth.

Dr McDonnell said it was much harder to perform caesareans on extremely obese women. ''It's incredibly complex in some of these women to get good access to their abdomen and then they have a much higher risk of wound infections afterwards as well,'' he said.

Toilets and beds were also needing to be modified to make sure they could handle the weight of extremely obese people, he said. Dr McDonnell is presenting the data at the Australian and New Zealand College of Anaesthetists meeting in Perth today.