PREGNANT women who consume trans fats from snacks and fast food may give birth to bigger babies.
A new study, reported in the American Journal of Clinical Nutrition, centred on nearly 1,400 pregnant women.
It found that the higher the woman's intake of trans fats during the second trimester of pregnancy, the larger her newborn was.
The study did not prove that trans fats alone boost foetal growth, and if they did, it is unclear how harmful that could be. But there are risks to having a larger-than-normal newborn, said lead researcher Juliana Cohen, of the Harvard School of Public Health in Boston, USA.
Big babies may have to be delivered by Caesarean-section and studies have found that they may have increased risks of diabetes and heart disease later in life, Cohen added.
"It's prudent to limit trans fats in your diet anyway. Pregnant women may want to think about how they could affect foetal growth as well," she said.
Artificial trans fats are found in foods that contain partially hydrogenated oils, including many baked and fried packaged foods like crisps and biscuits as well as fast food.
Some meat and dairy products contain natural trans fats, but people get the bulk of their trans fat in the artificial form -- although food producers and some restaurants have been cutting back after poor publicity.
The latest study was based on about 1,400 Boston-area women who gave birth between 1999 and 2002. The women completed dietary questionnaires during their first and second trimesters.
The relationship between higher trans fat intake and birth size held even after accounting for factors such as pre-pregnancy body weight, income, education and calorie intake.
They calculated that for every 1pc increase in trans fat as a replacement for carbohydrates in a woman's daily diet, her baby's foetal growth "Z score" -- which takes into account a newborn's birth weight and the week of pregnancy during which the baby was born -- inched up slightly.
Cohen said that while the effects of this would likely be limited in later life, the foods that contain trans fats were best limited for overall health.
Unfortunately, she added, these are sorts of foods that pregnant women often crave.