Women trying to become pregnant should avoid a diet high in junk food because it is linked with an increased risk of giving birth prematurely, scientists are warning.
Large amounts of fat, sugar and takeaway foods have been shown, for the first time, to increase the risk of a baby arriving early.
Mothers-to-be were warned to change their diets before they get pregnant to focus on vegetables, fish and fruit.
It is well known that a poor diet in pregnancy leads to poorer outcomes for the mother and baby, but researchers in Australia have now found that diet before conception plays a part, too.
Researchers at the Robinson Research Institute at the University of Adelaide investigated the dietary patterns of more than 300 south Australian women in the year before they conceived.
Dr Jessica Grieger, a postdoctoral research fellow at the institute, and lead author of the report, said: "Preterm birth is a leading cause of infant disease and death, and occurs in approximately one in 10 pregnancies globally.
"Anything we can do to better understand the conditions that lead to preterm birth will be important in helping to improve survival and long-term health outcomes for children.
"In our study, women who ate protein-rich foods, including lean meats, fish and chicken, as well as fruit, whole grains and vegetables, had a significantly lower risk of preterm birth."
She said that, by contrast, women who consumed mainly "discretionary" foods, such as takeaways, chips, cakes, biscuits and other foods high in saturated fat and sugar, were more likely to have babies that were born preterm.
"It is important to consume a healthy diet before as well as during pregnancy to support the best outcomes for the mum and baby," she said.
Babies born prematurely are at greater risk of suffering cerebral palsy, breathing difficulties, deafness and blindness.
Cathy Warwick, general secretary of the Royal College of Midwives, said the research proved the importance of giving women and girls health information even when they were not pregnant.
"It is important that we get these messages out to schools and colleges and women at every opportunity, in anticipation of pregnancy," she said.