Women who are obese during pregnancy expose their unborn children to a greater risk of stillbirth than they would if they smoked or drank alcohol.
Clinicians at the Canberra Hospital are concerned at the number of maternity patients who have significant weight problems.
It has been estimated that between 15 and 20 per cent of the hospital's maternity patients were obese and staff saw between 40 and 50 ''very large'' pregnant women last year.
Steven Adair, the hospital's clinical director, obstetrics and gynaecology, said many pregnant women were unaware of the risks excessive weight could pose to their own health and to their babies.
''Some of the published stillbirth data shows that obesity is a greater risk for stillbirth than smoking is,'' Dr Adair said.
''Yet, if somebody gets pregnant and they smoke they're more likely to stop smoking, if somebody drinks alcohol they're more likely to stop drinking during pregnancy.
''Yet being obese is more likely to be related to stillbirth than alcohol use in pregnancy.''
Dr Adair said overweight women should seek medical advice about improving their health before trying to fall pregnant.
''The current guidelines are to try and maximise weight reduction before the onset of pregnancy because that will increase your fertility rates and reduce your complication rates,'' he said.
Pregnant women who were obese or morbidly obese had an increased risk of pre-eclampsia, gestational diabetes, caesarean section or instrumental deliveries, hemorrhaging after delivery and giving birth to babies with birth defects. Chris Nolan, director of ACT Diabetes Services, said his staff saw two to three pregnant women each week with established Type 2 diabetes. Professor Nolan said the pregnancies of women with excessive weight or diabetes needed to be carefully managed.
Pregnancy outcomes for women with diabetes had improved since the 1970s thanks to research.
Similar research projects were now being established to try to improve outcomes for obese women.
The hospital sometimes saw pregnant women who had health risks linked to being underweight. But this was rare because being underweight tended to lower women's fertility.