Dieting in pregnancy is not only safe for the mother and baby, it can also reduce the risk of pregnancy complication compared with exercise alone or a combination of both exercise and dieting, according to a new analysis of past research.

Researchers reporting in the British Medical Journal looked at results from 44 previous studies, which in involved a total of 7,278 pregnant women.

They found that those who were on a healthy, calorie-controlled diet prevented excess weight gain and decreased the risk of pregnancy complications like pre-eclampsia, diabetes, high blood pressure and early delivery, compared to expectant mothers who ate for two.

The findings are especially true for pregnant women who are overweight or obese.

Past studies have linked obesity during pregnancy to an increased risk of developing high blood pressure, diabetes, miscarriage, birth defects, blood clots, pre-eclampsia, and even maternal and infant deaths.

Researchers from the Queen Mary, University of London compared the effect of diet, exercise or a combination of the two on the health of the mother and the baby and studied the relationship between weight gain and pregnancy complications in both the mother and the baby.

While all three weight reduction methods reduced the mother’s weight gain, researchers found that dieting alone reduced the most weight in pregnant women, with an average weight loss of nearly 4kg (8.8 lbs). Exercise only resulted in an average weight reduction of just 0.7kg (1.5 lbs) and a combination of diet and exercise resulted in an average weight loss of one kilogram (2.2 lbs).

The study also found that women who followed a calorie controlled diet had were 33 percent less likely to develop pre-eclampsia or hypertension in pregnancy, were 60 percent less likely of developing gestational diabetes, were 70 percent less likely to have gestational high blood pressure and 32 percent less likely of having an early delivery.

Researchers stressed that infants’ birth weights were not affected by dieting.

According to a 2009 study published in the Maternal and Child Health Journal, nearly half of American women who are of child bearing age are obese, and 20 percent to 40 percent of women gain more than the recommended weight during pregnancy.

"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," lead researcher Dr. Shakila Thangaratinam, a consultant obstetrician at Queen Mary, said in a statement.

"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 said. "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."

Researchers said that the dietary advice was based on limiting overall calorie consumption, balancing protein, carbohydrate and fat and eating more whole grains, fruits, vegetable and pulses like beans and lentils.

"What we don't know is why diet should be so much better than exercise in controlling weight gain. It could be that it is simpler and easier for women to stick to. It may also be that eating a high-fibre diet has other positive health effects for a pregnant woman," Thangaratinam added.

However in an accompanying editorial, Lucilla Poston and Lucy Chappell of St. Thomas' Hospital in London wrote that there is still not enough evidence to support dietary or any other type of intervention during pregnancy.

"At a time when more than half the women of reproductive age in the United Kingdom are overweight or obese, any analysis of weight management interventions in pregnancy is timely and welcome," they wrote. However they noted that the study does not provide enough evidence to reconsider the current guidelines for weight management in pregnancy.

However they added that several ongoing studies may provide valuable insight into effective ways to prevent excessive weight gain during pregnancy.

Experts say that weight gain during pregnancy is normal, but obese women should not gain more than 15 pounds, which only half the amount recommended for women of normal weight, according to the Mayo Clinic.

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