Weight management 'benefits' for mother and baby
Dieting in pregnancy is safe for women and does not carry risks for the baby, a review of research has suggested.
The British Medical Journal analysis looked at the findings from 44 previous studies involving more than 7,000 women.
The London-based team said following a healthy diet - and not eating for two - prevents excess weight gain and cuts the risk of complications.
But current guidelines do not advocate dieting or weight monitoring.
The advice from the National Institute for Health and Clinical Excellence (NICE), published in 2010, says: "Dieting during pregnancy is not recommended as it may harm the health of the unborn child."
However women are advised to aim to reach a healthy weight before conceiving.
Babies' weights 'unaffected'
Half the UK population are either overweight or obese and the rates are rising.
And in Europe and the US, between 20% and 40% of women gain more than the recommended weight during pregnancy.
High weights are linked to complications such as pre-eclampsia, diabetes and high blood pressure as well as early delivery.
This review, funded by the National Institute of Health Research (NIHR), compared diet, exercise or a combination of the two.
Dietary advice was based on limiting calorie intake, having a balanced diet and eating foods such as whole grains, fruits, vegetables and pulses.
The researchers then examined how much weight women gained during their pregnancies and if there were complications.
While each approach reduced a woman's weight gain, diet had the greatest effect with an average reduction of nearly 4kg (8.8lbs).
With exercise, the average reduction in weight gain was just 0.7kg (1.5lbs). A combination of diet and exercise led to an average reduction of 1kg (2.2lbs).
Women following a calorie-controlled diet were significantly less likely to develop each of the complications considered, but the researchers say those findings need to be repeated in larger studies.
Babies' birth weights were not affected by dieting.
'Simpler and easier'
Dr Shakila Thangaratinam, a consultant obstetrician at Queen Mary, University of London who led the study, said: "We are seeing more and more women who gain excess weight when they are pregnant and we know these women and their babies are at increased risk of complications.
"Weight control is difficult but this study shows that by carefully advising women on weight management methods, especially diet, we can reduce weight gain during pregnancy.
"It also shows that following a controlled diet has the potential to reduce the risk of a number of pregnancy complications."
She added: "Women may be concerned that dieting during pregnancy could have a negative impact on their babies. This research is reassuring because it showed that dieting is safe and that the baby's weight isn't affected."
But in a commentary in the journal, women's health experts from St Thomas' Hospital in London - including Lucilla Poston who helped develop the NICE guidance, said it would be "premature" for the current guidance, which only recommends women be weighed at their first pregnancy check-up, to change.
Dr Janine Stockdale, research fellow at the Royal College of Midwives, said: "We should be careful to note that the researchers are not advising women to lose weight during pregnancy; this is about managing excessive weight or weight gain.
"If a woman is on target to gain the right amount of weight during her pregnancy, then 'dieting' and 'calorie-controlled dieting' as we commonly understand these terms, is not for her.
"We need to reassure women that under the care of a midwife or other health professional, weight management is safe."