Nearly two-thirds of U.S. women of childbearing age are overweight -- and almost half of those women are obese. With a record number of overweight and obese American women getting pregnant, the health stakes of gaining too much weight during pregnancy for both children and mothers are getting increasingly higher, according to a new study.

Kathleen Rasmussen, Cornell professor of nutritional sciences, led the 22-month-long study to review and update the 1990 Institute of Medicine recommendations for weight gain during pregnancy. The study found that up to 73 percent of U.S. women fail to gain an amount of weight within the recommended guidelines, with most overweight women gaining too much. Yet a high proportion of women get little or no advice on how much weight to gain during pregnancy.

A report that Rasmussen has co-authored for the Institute of Medicine and the National Research Council spells out the updated guidelines. They stress the need for all obese and overweight women to be counseled before pregnancy to learn how to reach a healthy weight before conception.

"This may reduce their risk during pregnancy and normalize infant birth weight as well as improve their long-term health," Rasmussen says.

The report also stresses that all women should be counseled about their weight during and after pregnancy. Implementing these counseling guidelines, she says, "would represent a radical change in the care provided to women of childbearing age."

According to the report, women who gain more weight than is recommended are at greater risk for a host of pregnancy-associated health problems, including excessive postpartum weight retention and possibly subsequent maternal obesity, which are linked to yet more health problems.

The new federal guidelines remain the same for weight gain during pregnancy for underweight, normal weight and overweight women, as defined by body mass index (BMI), a calculation based on the relationship of weight to height. However, the BMI cutoffs to define these groupings have changed. In the past, normal was defined with a BMI of 19.8 to 26; the new guidelines for normal women are stricter at 18.5 to 24.9. The guidelines recommend that underweight women (BMI of less than 18.5) gain 28-40 pounds, normal-weight women 15-25 pounds and overweight women (BMI of 25-29.9) 15-25 pounds.

But obese women, those with BMIs greater than 30, should gain only 11-20 pounds; this guideline establishes a new upper limit for this group of women.

The new guidelines also include a relatively narrow range of recommended gain for obese women.

Rasmussen stresses that even though the new weight guidelines may not be significantly different than the Institute of Medicine's 1990 guidelines, "they are derived from a completely different process that involved assessment of the risk -- not just to infants -- but to both mothers and their infants."

Individuals can determine their BMI using this online calculator: http://www.nhlbisupport.com/bmi/. The full report can be found at the National Academies Press at http://www.national-academies.org/.