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"This is a concern because gaining too much weight has health consequences for both mothers and infants," said study co-author Andrea Sharma, an epidemiologist with the U.S. Centers for Disease Control and Prevention's Maternal and Infant Health Branch.
Some weight gain might result from misconceptions, said Dr. Karen Cooper, an obstetrician/gynecologist and director of the Cleveland Clinic's Be Well Moms program.
"Most women feel that pregnancy is the time when weight does not matter and it is an opportunity to eat as much as desired," said Cooper, who was not involved with the new study. "Most believe the myth that the weight will be lost quickly and easily after delivery."
For the study, published in the April print issue of the journal Obstetrics & Gynecology, Sharma's team analyzed information on more than 44,000 women who gave birth to a single full-term baby in 2010 or 2011. The women were from 28 states.
Women were classified based on their body mass index (BMI), a ratio of height to weight. A BMI between 18.5 and 25 is considered normal weight; below 18.5 is underweight, between 25 and 30 is overweight, and over 30 is obese. Those with the greatest obesity had a BMI of 35 or greater.
Whether women gained too little, too much or just the right amount of weight during pregnancy depended largely on their pre-pregnancy weight, the researchers found.
One in five gained too little weight during their pregnancy, and just 32 percent gained an amount that fell within the recommendations of the Institute of Medicine. Slightly more than 47 percent of the women gained excessive weight, according to the study authors.
Institute of Medicine guidelines recommend gaining 25 to 35 pounds if normal weight at the start of pregnancy; 28 to 40 pounds if underweight; 15 to 25 pounds if overweight; and 11 to 20 pounds if obese at the start of pregnancy.
Women who were overweight or obese before pregnancy were two to three times more likely to gain more weight than recommended, compared with those of normal weight. Yet women with the most obesity were also nearly twice as likely to gain too little weight, the study found.
Race and ethnicity, level of education and health conditions such as diabetes or nausea during pregnancy also affected how much weight the women gained. Women who had high blood pressure conditions (hypertension), who reported regular pre-pregnancy physical activity or who quit smoking while pregnant were more likely to gain too much weight.
Experts are not sure why women struggle to stay within the weight gain guidelines during pregnancy, but there are several possible factors at play, said Dr. Megan Lutz, an ob-gyn at the Cleveland Clinic.
"Women probably don't know what risk factors they may have at the start of pregnancy which put them at risk for excessive or inadequate weight gain in pregnancy," Lutz said. "Women may not know dietary or exercise recommendations to succeed in pregnancy."
Choosing healthy portions of high-protein, low-fat foods -- such as chicken breast, turkey, yogurt, nuts, lentils, chia seeds, avocado and egg whites -- is a way to help control weight gain, especially when women crave carbohydrates or unhealthy foods, Lutz said.
Tips for losing weight after giving birth include breast-feeding and exercising at least four times a week at a moderate to high intensity, Cooper said. Those who don't lose the weight will be heavier with the next pregnancy, she added.
Women who gain excess weight are more likely to have larger-than-normal newborns, who then have a higher risk of childhood obesity, according to background information in the study. Even without additional pregnancies, women who don't lose post-pregnancy weight have a higher risk of type 2 diabetes and high blood pressure, Cooper said.
Dieting is not recommended during pregnancy, Sharma said, but women can track their weight to determine if they should modify their eating and physical activity levels during pregnancy.
"Women should talk with their doctor about their weight gain goals, strive to maintain a healthy lifestyle and remember pregnancy is not eating for two," Sharma said.
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