The common belief that pregnant women can eat for two has been scotched by research suggesting dieting during pregnancy can be beneficial.
Experts found that weight management was not only safe but could also reduce complications for pregnant women and be advantageous to the baby.
The risk of pre-eclampsia - which causes high blood pressure - diabetes and premature birth can all be reduced if the mother-to-be sticks to a healthy, calorie-controlled diet, the study published on bmj.com found.
In contrast, excessive weight gain during pregnancy was linked to a number of serious health problems.
In the UK, more than half of women of reproductive age are said to be overweight or obese, and across Europe and the US up to 40% of women gain more than the recommended weight in pregnancy.
But the team of researchers from Queen Mary, University of London, who carried out the study found weight management interventions in pregnancy were effective in reducing weight gain in the mother.
Dietary intervention resulted in the largest average reduction in weight gain (almost 4kg) compared with 0.7kg for exercise and 1kg for a combination of the two.
Diet also offered the most benefit in preventing pregnancy complications, the study found.
Researchers concluded: "Dietary intervention is effective, safe and potentially cost effective and dominates physical activity-based intervention."
But experts at St Thomas' Hospital in London suggested there was not yet sufficient evidence to support any particular intervention.
Lucilla Poston, director of the maternal and fetal research unit, and Lucy Chappell, clinical senior lecturer in maternal and fetal medicine, said it would be "premature" for the National Institute for Health and Clinical Excellence (NICE) to reassess its guidelines, which do not advise regular weighing of pregnant women.
The researchers analysed the results of 44 randomised controlled trials involving more than 7,000 women.