A pregnant woman eats for two, as the saying goes. But in the United States, a lot of them are eating for three or more.

A new study from the Centers for Disease Control and Prevention finds that nearly half of the pregnant women in American gain too much weight during their pregnancies. Women who are overweight or obese before they become pregnant are most likely to add too many additional pounds during those crucial nine months.

The Institute of Medicine recommends that women with a body mass index in the normal range (between 18.5 and 24.9) gain 25 to 35 pounds during the course of a pregnancy. Women who start out underweight (with a BMI below 18.5) should put on 28 to 40 pounds while pregnant; women who are overweight should add 15 to 25 pounds; and obese women should gain only 11 to 20 pounds during their pregnancies, the guidelines say.

For their study, researchers from the CDC’s Division of Reproductive Health examined birth certificates from 46 states plus the District of Columbia. That gave them information on the mothers’ height and weight before and after their pregnancies. The researchers focused their analysis on singleton births that were carried to full term (at least 37 weeks of gestation). Their final data set covered nearly 80 percent of all pregnant women in America.

In every single state examined, the number of pregnant women who gained too much weight was higher than the number who gained too little or the right amount, the researchers found.

Nationwide, 47.5 percent of women had excessive weight gain during their pregnancies, with individual states ranging from a low of 38.2 percent in New Jersey to a high of 54.7 percent in Missouri. But Missouri wasn’t that much of an outlier — in 17 states, more than half of pregnant women gained too much weight.

Excessive weight gain was a problem for women across the BMI spectrum, affecting 23.5 percent of women who were underweight at the start of their pregnancies and 37.6 percent of women who had a normal weight before they got pregnant. But it was especially rampant among women who were overweight or obese — 61.6 percent and 55.8 percent of them, respectively, added too many extra pounds during nine months of pregnancy.

Inadequate weight gain was a problem too, affecting 20.4 percent of pregnant women nationwide. In 20 states, at least one in five mothers-to-be put on too few pounds. The highest prevalence was in Georgia, where 25.5 percent of pregnant women failed to gain enough weight.

That left only 32.1 percent of pregnant women who hit the Institute of Medicine target for gestational weight gain. New Jersey had the highest proportion of women in this category (39 percent) and Alaska had the lowest (26.2 percent).

The study results appear in Thursday’s edition of the Morbidity and Mortality Weekly Report.

Making sure women gain the right amount of weight during pregnancy is important for both mother and baby. Gaining too little weight puts babies at risk of having a low birthweight, which increases the baby’s risk of diabetes, heart disease, high blood pressure and obesity later in life. On the flip side, babies born to moms who gain too much weight have an increased risk of childhood obesity and metabolic syndrome.

For women, excessive weight gain during pregnancy makes that “baby weight” more difficult to lose. It also makes obesity more likely.

The CDC authors wrote that excessive weight gain often starts early in pregnancy, so it’s never too early for obstetricians and their patients to start keeping track of their weight. Although some women are eager to start eating for two, in most cases it’s not necessary until the second trimester. From that point on, the demands of a growing baby require women to consume an additional 340 to 450 calories per day, according to the study.

Exercise is an important part of the equation, and pregnant women should log at least 150 minutes per week of brisk walking or another moderate-intensity activity, the researchers added.

“Some women might also believe that physical activity during pregnancy is risky,” they wrote. “However, physical activity is safe and recommended for most pregnant women and might reduce some pregnancy-related complications.”