(AP) -- 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 blood 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.
©2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Explore further: Pre-term birth and asthma: Preterm birth may increase the risk of asthma and wheezing disorders during childhood