Women who are skinny before pregnancy are supposed to gain 25 to 35 pounds, but the overweight should gain only 15 to 25 pounds, and the obese just 15 pounds.

Dieting during pregnancy is a big no-no -- it can harm the baby.

With obesity on the rise, new guidelines urge obstetricians and gynecologists to bring up the long-taboo subject before their patients ever become pregnant, and to take special steps when mothers-to-be are too overweight.

Among those steps: Nutritional counseling so they don't gain too much during pregnancy; earlier-than-normal testing for gestational diabetes; and consulting with an anesthesiologist about safe sedation well before the due date, in case an emergency Caesarean section is needed.

If there's ever a time women are going to improve their own health habits, it's during pregnancy, notes Dr. Laura Riley of Massachusetts General Hospital, who co-authored the guidelines published this month by the American College of Obstetricians and Gynecologists.

'Wasn't Hard to Eat Healthy'

Mothers-to-be are keenly aware that everything entering their bodies can go straight to their developing babies.

"Honestly, it was the one time in my life where it wasn't hard to eat healthy," said Kristin Noon-Batmaca of Amesbury, Mass., whose son, Teddy, was born two months ago.

Four years of infertility treatment had failed. Then Noon-Batmaca lost 80 pounds and conceived naturally -- and, under Riley's care, gained only a healthy fraction back while pregnant, by eating lots of fruits and vegetables.

"I'm not going to say there have not been people who left my office in tears, no question," says Riley, who carefully broaches obesity as a medical problem.

"They need to be retrained for their own health, but you know what? They're going to be raising kids. You don't want them to teach their kids those same bad eating habits," she adds.

Increases the Risk of Miscarriage

No one knows just how many pregnancies are complicated by obesity. The government estimates one-third of adult women in the United States are obese. Also, most of the estimated 110,000 people who each year undergo gastric bypass, the leading obesity surgery, are women in their child-bearing years -- who are supposed to delay conceiving for up to 18 months afterward.

Obesity increases the risk of miscarriage and can at least double the woman's chances of gestational diabetes or a condition called pre-eclampsia, which can be life-threatening to mother and fetus. It also doubles the baby's risk of a neural tube birth defect such as spina bifida, the obstetricians group found.

The more overweight a woman is, the more likely she'll need a C-section, yet the riskier C-sections become. Obese women are more likely to suffer excessive blood loss and infections.

Excess weight even makes it difficult to estimate the fetus' weight, track its heart rate, or administer an epidural or other anesthesia during labor.

That's a lot of pressure for an expecting mom. So the guidelines urge doctors to bring up weight before a woman conceives. Preconception visits today, instead, typically focus on such things as genetic risks.

Balancing Act

Once a woman is pregnant, the recommendations stress:

Post-surgery pregnancy risks still aren't clear, although some recent studies suggest women may do better than if they had remained obese. There are reports of gastrointestinal bleeding and other risks, especially if the woman got pregnant too soon.

Obesity in pregnancy is a balancing act, stresses Riley, who tells patients not to count every pound. "We're trying to help them be healthy about it, not crazy."