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(CBS News) Is it safe for a pregnant woman to go on a diet? According to a new study, not only is it safe, but it can even be beneficial and reduce the risk of dangerous complications.

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Doctors already know that piling on excess pounds during pregnancy increases the risk for complications such as pre-eclampsia and gestational diabetes, but many are cautious of giving weight loss advice out of concerns for adverse events that would harm a mom and baby's health.

For the new study, published in the May 18 issue of the British Medical Journal, researchers analyzed 44 relevant studies that comprised of more than 7,200 women, looking for the effects diet, exercise or both had during pregnancy. They looked specifically at how much weight women gained throughout pregnancy and whether a mother or child suffered any complications.

The researchers found that all three methods reduced a mother's weight, but diet showed the greatest effect with an average reduction of almost 9 pounds. Pregnant moms who only exercised lost about 1.5 pounds, and moms who did a combination of diet and exercise lost an average of 2.2 pounds.

The diets were tied to health benefits for pregnant moms, the researchers found. Women who went on a calorie-restricted diet were 33 percent less likely to develop pre-eclampsia, a spike in blood pressure caused by significant amounts of protein in the urine. Left untreated, the condition can be fatal to a mom and baby, according to the Mayo Clinic.

Mothers' risk of gestational diabetes was 60 percent lower with a calorie-controlled diet, and their risk of gestational high blood pressure was 70 percent lower, compared with the other groups. The risk of premature birth was also reduced 32 percent in dieting moms.

"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," lead researcher Dr. Shakila Thangaratinam, a clinical senior lecturer and consultant obstetrician at Queen Mary, University of London, said in a news release. She said it also shows that following a controlled diet has the potential to reduce the risk of a number of pregnancy complications.

Dietary advice in the study based on limiting overall calorie intake, balancing protein, carbohydrates and fat, and eating foods such as whole grains, fruits, vegetables and legumes. The research did not show what a healthy amount of weight gain was for pregnant women or how many calories a woman should eat daily during pregnancy.

The March of Dimes says women beginning pregnancy at a healthy weight should gain 25 to 35 pounds over the nine-month pregnancy, and women who are obese should only gain 11 to 20 pounds during pregnancy.

"What we don't know is why diet should be so much better than exercise in controlling weight gain," Thangaratinam said. "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."

In an accompanying editorial in the same journal, Lucilla Poston, head of women's health at King's College London and Lucy Chappell, clinical senior lecturer in maternal and fetal medicine at King's, wrote a study such as this was "timely and welcome" because in the U.K. half of women of reproductive age are overweight and obese. But they said the study's evidence was too limited to offer advice to women.

"There is not yet sufficient evidence to support any particular intervention," they wrote.

According to the CDC, obesity during pregnancy affects about one of five pregnant women in the U.S. and in addition to the previously mentioned complications, the excess weight gain also leads to more use of inpatient and outpatient health care services, longer hospital stays, and more time spent with a doctor.