WASHINGTON Eating for two? New guidelines are setting how much weight women should gain during pregnancy surprisingly little if they're already overweight or obese when they conceive.

The most important message: Get to a healthy weight before you conceive, say the Institute of Medicine's guidelines, the first national recommendations on pregnancy weight since 1990. It's healthiest for the mother less chance of pregnancy-related high *lo** pressure or diabetes, or the need for a C-section and it's best for the baby, too. Babies born to overweight mothers have a greater risk of premature birth and becoming overweight themselves, among other concerns.

That's a tall order, considering that about 55 percent of women of childbearing age are overweight and preconception care isn't that common.

Once a woman's pregnant, the guidelines issued Thursday aren't too different from what obstetricians already recommend but they're not easy, considering about half of women fail to follow them today.

Among the advice:

_A normal-weight woman, as measured by BMI or body mass index, should gain between 25 and 35 pounds during pregnancy. A normal BMI, a measure of weight for height, is between 18.5 and 24.9.

_An overweight woman BMI 25 to 29.9 should gain 15 to 25 pounds during pregnancy.

_An obese woman BMI of 30 or higher should gain 11 to 20 pounds. This marks the first recommendation ever set for women so heavy.

_An underweight woman BMI less than 18.5 should gain 28 to 40 pounds.

What if a mom-to-be has already gained too much? On average, overweight and obese women already are gaining five more pounds than the upper limit.

But pregnancy is not a time to lose weight, stressed guidelines co-author Dr. Anna Maria Siega-Riz of the University of North Carolina, Chapel Hill.

"It's not, 'Hey you gained enough, now you need to stop,'" Siega-Riz said. "Let's take stock of where you're at and start gaining correctly."

Indeed, the guidelines lay out that in the second and third trimesters, underweight and normal-weight mothers should be putting on a pound a week for proper fetal growth. The overweight and obese need about half a pound a week.

Hopping on the scale during prenatal checkups makes for a sensitive moment, especially in a culture that cherishes the ice cream-and-pickles stereotype.

Implementing the guidelines may take a move "to change the whole culture about pregnancy" and eating, Siega-Riz said. She noted that in studies of the overweight, "most of these women will tell you that they've never been told how much weight to gain" during pregnancy.