The study, which reviews 44 previous studies, says following a calorie controlled diet can reduce the risk of serious complications.
Current guidelines do not advocate dieting while pregnant, but the new research says watching the calories can reduce the risk of pre-eclampsia, premature birth and diabetes.
Dr Janine Stockdale, RCM research fellow, said: 'This is welcome research which will underpin the advice and support that midwives are already giving to women.
'Good nutrition before and during pregnancy is hugely important to help ensure good health for the mother and her baby. This can also have benefits well beyond the pregnancy itself.
'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.'
She stressed that if a woman is on target to gain the right amount of weight, then dieting would not be the correct thing for her to do.
She added: 'We need to reassure women that under the care of a midwife or other health professional, weight management is safe and offers benefits to mother and baby, such as a reduced risk of pregnancy-related diabetes and pre-term birth.'
The review was funded by the National Institute of Health Research
and 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 examined how much weight women gained during their pregnancies and whether there were complications.
They found women who followed a calorie-controlled diet were significantly less likely to develop complications and that birth weights were not affected.
Slimming World, the UK's leading slimming club and RCM Alliance
partner, has welcomed the news.
Dr Jacquie Lavin, head of nutrition and research, said: 'We’re delighted by this review of the latest evidence which shows that by preventing excess weight gain, women can reduce their risk of complications in pregnancy.
'It’s not about crash diets or celebrity fads, but about seeking healthy lifestyle support that will empower them to make healthy choices so that they get all the necessary vitamins and nutrients.'
She added: 'It’s advisable for pregnant women to speak to their midwife about the best way to safely manage their weight.'
The researchers stress that further research and larger studies looking at the topic are needed.
The research is published today (18 May) in the BMJ
and can be read here