A new study finds that young, low-income women often gain too much weight during pregnancy, raising concerns about the potential long-term impact on their obesity risk.

Nearly two-thirds of 427 pregnant women, mostly black or Hispanic, seen at two U.S. medical clinics put on more than the recommended weight during pregnancy.

And a year after giving birth, about half had retained at least 10 of their pregnancy pounds.

Writing in the American Journal of Obstetrics & Gynecology, Dr. Bonnie E. Gould Rothberg of Yale University School of Medicine in New Haven, Connecticut, and colleagues say the findings are worrisome.

Excess weight gain during pregnancy increases the odds of having a larger-than-normal baby and needing a C-section, for example. And women are less likely to be able to shed their pregnancy pounds afterward, raising their risk of diabetes and high blood pressure.

Studies also hint that large newborns are more likely to become overweight themselves down the road.

The researchers found that before pregnancy, just shy of a quarter of the women were overweight, and slightly more were obese.

Among women who were normal-weight before pregnancy, four in 10 fit the definition of “overweight” one year after giving birth, and one in 20 fell into the obese category.

Of those who were overweight before becoming pregnant, more than half were obese one year after delivery, based on body mass index (BMI), a measure of weight in relation to height.

A BMI above 25 is considered overweight and above 30, obese.

One of the concerns with these weight changes is that they resulted from just one pregnancy, Gould and her colleagues note.

Since the women in the study were only between the ages of 14 and 25, the researchers point out, it is likely that they will have more children, and possibly retain more weight after those pregnancies.

The Institute of Medicine (IOM), an advisory body to the U.S. government, recommends that normal-weight women gain between 25 and 35 pounds during pregnancy. Underweight women are advised to gain 28 to 40 pounds, while those who are overweight should put on 15 to 25 pounds.

Obese women are given the narrowest weight-gain range, with 11 to 20 pounds considered optimal.

That latter guideline was newly issued last year, in recognition of the fact that since the 1980s, there has been an increase in the number of U.S. women who are obese before becoming pregnant.

Few studies have examined patterns of weight gain among young minority women in the U.S., according to Gould Rothberg and her colleagues, who call for more efforts to make such women meet IOM recommendations.

The researchers did find that certain factors were related to greater weight loss after pregnancy. Women who breastfed, for instance, lost about 1.5 pounds (0.68 kilograms) more per week than those who bottle-fed.

While not all past studies have come to the same conclusion, Gould Rothberg and her colleagues say their findings mean it might be worth promoting breastfeeding among young, low-income mothers.

They also found that Hispanic women were generally more successful than either white or black women at losing weight after delivery.

Further research should focus on why some women gain too much poundage during pregnancy, the team says, and why some shed the extra pounds fast why others never do.

SOURCE: American Journal of Obstetrics & Gynecology, online October 25, 2010.