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Nearly half of U.S. women gain too much weight while they’re pregnant, and another 20 percent don’t put on enough, federal health experts reported Thursday.

Fewer than a third of pregnant women put on the right amount of weight – something that’s important for both mother and baby, the team at the Centers for Disease Control and Prevention found.

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Overweight and obese women most commonly exceeded the guidelines.

Women need to eat extra calories during pregnancy, although not that much - only about 350 to 450 extra calories during the second and third trimester, said CDC’s Andrea Sharma, who worked on the study.

"It's not about eating twice as much. It's eating twice as healthy," she said.

How much moms should gain depends on their weight when they become pregnant.

Experts say women of normal weight should add 25 to 35 pounds. Overweight women should gain 15 to 25, and obese women should only add 10 to 20. For unusually thin women, weight gain should be about 30 to 40 pounds.

Gaining too little weight increases the risk that the baby will be born very small. Too much weight can lead to obesity and other health problems for the mom. It can lead to dangerous complications during labor and delivery. And it raises the risk the baby will become obese, diabetic, and have other problems later in life.

The study involved more than 3 million U.S. pregnant women during 2012 and 2013. In most of the 46 states included, birth certificates now also record pre-pregnancy weights.