A series of articles published in the Medical Journal of Australia draws attention to the challenges presented by overweight and obesity to people in a variety of age groups.

A study of data from Brisbane's Mater Hospital showed that between 1998 and 2009 there was a threefold increase in the proportion of women with single pregnancies who were obese.

Increasing maternal body mass index was associated with adverse pregnancy outcomes such as hypertension in pregnancy, gestational diabetes, caesarian deliveries, stillbirth and neonatal death.

It was also associated with larger babies being born and health problems for babies such as hypoglycaemia, jaundice, respiratory distress and the need for intensive care.

''Most associations remained significant after adjusting for maternal age, parity, insurance status, smoking status, ethnicity and year of the birth,'' the authors wrote.

While obesity in pregnancy had not reached epidemic levels, the hospital was dealing with eight or nine births by obese women each week.

Another study of students aged between 12 and 17 years who participated in a national diet and exercise survey found that 18 per cent were overweight and 5 per cent were obese.

''These findings suggest the preventative measures should include a particular focus on facilitating physical activity and reducing sedentary behaviour, as well as promoting adequate sleep, particularly among young people from lower [socio-economic position] neighbourhoods who appear most susceptible,'' the authors wrote.

A third study called for more research into how much physical activity preschoolers needed, amid reports that most Australian preschoolers were falling well short of a national recommendation of at least three hours of exercise each day.

Associate Professor Helen Skouteris, of Deakin University, said it was difficult to ascertain whether preschoolers were sufficiently active.

She said different activity guidelines were needed for children of different age groups.

''We argue that age groupings are needed because toddlers and preschoolers, school-aged children and adolescents are physiologically and developmentally different from each other,'' Professor Skouteris said.

''These developmental differences need to inform age-specific recommendations.''

Associate Professor Anthea Magarey, of Flinders University, said health professionals should promote the importance of healthy eating and activity and an awareness of healthy growth, especially in young children.

''Obesity prevention management is complex and requires complex solutions, but physical activity is an important component for all ages and, like healthy eating, has much broader health outcomes than weight management alone,'' Associate Professor Magarey said.