Women may need weight loss counselling, nutrition and exercise advice before they conceive since weight loss isn't an option during pregnancy. (CBC)

Women should aim to reach a healthy weight before they conceive, according to new U.S. guidelines that highlight the risks of carrying excess pounds for moms and for newborns.

The Institute of Medicine, an independent organization that advises the U.S. government on health, released the updated guidelines on Thursday to address needs the needs of two-thirds of the population who are overweight or obese.

"It had become clear that heavier women could gain less weight and still deliver an infant of good size," the report said.

The health risks from excess weight include greater chances of a caesarean section or high blood pressure for mom and birth defects and greater risk of childhood obesity for the baby.

The new guidelines add a fourth category, obese, to the recommendations on how much weight women should gain over the course of a pregnancy.

Women are obese should gain about 4.9 kilograms to 9.1 kilograms while pregnant, the expert panel concluded.

The advice includes:

• A normal-weight woman, as measured by body mass index (BMI), should gain between 11.3 kilograms and 15.8 kilograms 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 6.8 kilograms to 11.3 kilograms during pregnancy.
• An obese woman — BMI of 30 or higher — should gain 4.9 kilograms to 9.1 kilograms.
• An underweight woman — BMI less than 18.5 — should gain 12.7 kilograms to 18.1 kilograms.

For most women, the total calories they should add to their diet during the last half of pregnancy is 300 calories, which is equivalent to one large glass of milk and a piece of fruit.

"Attaining those guidelines or the recommendations in those guidelines will be difficult for a lot of women," acknowledged Dr. Sarah McDonald, a professor of obstetrics and gynecology at McMaster University in Hamilton.

The guidelines stress the need to give weight loss counselling, nutrition and exercise advice to women before they conceive to ensure healthier babies.

Change culture of pregnancy

Pregnancy is not the time to lose weight, said Dr. Anna Maria Siega-Riz of the University of North Carolina, Chapel Hill, who co-authored the guidelines.

"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."

The culture of pregnancy needs to change, given that in studies of the overweight, most women said they were never told how much weight to gain during pregnancy, Siega-Riz added.

Health Canada is currently reviewing its guidelines and plans to release an update in the fall. Currently Canadian guidelines reflect the institute's previous recommendations from 1990, before the obesity epidemic occurred.

With files from The Associated Press