ABOUT 50 per cent of women in Australia are overweight or obese when they fall pregnant, and researchers have found the diet of those women gets progressively worse during pregnancy and is especially poor after giving birth.
The University of Adelaide study of nearly 300 overweight or obese women - defined as those with a body mass index of more than 25 - is the first to examine the diets of pregnant and overweight women until after birth.
The lead author, Lisa Moran, said, while some women suffered from morning sickness and food aversion, more than three quarters of the women entered the study in their second trimester, by which time those ailments usually had passed.
''It may be that for some women morning sickness doesn't resolve as quickly,'' said Dr Moran, from the university's medical research centre, the Robinson Institute. ''But some women may still feel that pregnancy is the one time where they can relax some of their previous eating habits, and the idea of eating for two does seem to still resonate.''
Diet might also depend on family, friends and culture.
Participants completed a food frequency survey at four points throughout the study, published in the International Journal of Obesity, the last four months after birth. Thirty per cent had a poor diet at the start, but this jumped to nearly half after birth, with fruit, vegetable and dairy intake all poor. Energy intake from added sugars increased after birth, as did alcohol consumption.
More than 40 per cent did not consume enough iron or calcium while pregnant. Diet was also poorer as social disadvantage increased, and some women may have deliberately avoided meat and seafood because of bacteria fears.
Dr Moran said some health professionals may be reluctant to provide dietary advice that was too restrictive because they feared some might take it ''to the extreme of dieting - something we don't want''.
But Kyra Sim, from the Boden Institute of Obesity, Nutrition, Exercise and Eating at Sydney University, said it was most important that women ensured they were healthy before attempting to get pregnant.
''While women may make changes when first getting pregnant to improve their health, we know maintaining those changes is the hardest,'' Dr Kim said.
''We also know women tend to gain weight as they get older and are falling pregnant later, which could also be a factor.''