Obstetricians are often in the difficult position of reassuring women that some weight gain — but not too much — is necessary and beneficial. Obesity is an issue, but so are eating disorders.

"The pregnant body is pretty smart," said Dr. Nancy O'Neil, an obstetrician/gynecologist with Northwest Women's Healthcare, which is affiliated with Swedish Medical Center. "The body wants a certain percent of body fat by 20 weeks, and it will work hard to get there."

The general guidelines of a 25- to 35-pound gain during pregnancy are for an average-weight woman. "A woman who is very slender when she gets pregnant may gain more weight in the beginning, then plateau," O'Neil said. "An overweight woman might not gain much since she already has fat there."

A review of 1990s research concluded that weight gains within the recommended range are associated with better pregnancy outcomes, according to a 2000 issue of the American Journal of Clinical Nutrition. Too little gain may result in babies with low birth weight, while women on the upper end may have very large infants with a higher risk of birth injury and other problems.

Research published in 1996, though, found less than a third of women gain weight within the current guidelines; other studies show the amount of total weight gain varies widely among women with healthy babies.

Postpartum

Women should give themselves six months to a year to get back to their pre-baby weight, experts say. Studies found that pregnant women who gained more than the recommended amount were twice as likely to retain excessive weight as those who didn't.

Numerous studies on body-weight changes all reported postpartum weight increases, according to a 2002 article in the Healthy Weight Journal. After six months to two years, studies found a 2- to 7-pound gain in white women and 7 to 13 pounds in black women.

Though there are many benefits to breastfeeding, the oft-cited component of faster weight loss may not be one of them. A review of nine studies in a 2002 Healthy Weight Journal article noted that "the effects of lactation on postpartum weight are not yet clear."

Researchers at the Cincinnati Children's Hospital Medical Center found that moms who didn't breastfeed lost fat faster than nursing women during the first six months, though both shed about the same amount of weight.

Doctors should be careful not to give nursing moms false expectations, the researchers concluded in a 2004 issue of the American Journal of Clinical Nutrition.

"A woman with a 2-month-old baby who's very slender again is genetically blessed," O'Neil said. "Most women's bodies try to hold onto the weight while they're breastfeeding. A lot of women really fight that.

"Most women, for the first six weeks, are too tired and too absorbed with their baby to exercise much," O'Neil said. "But by six weeks, the societal pressures to be thin again are very, very strong."

Pushing the body to exercise immediately won't do much good anyway, said O'Neil, a mom who has delivered babies for 23 years. The abdominal muscles, for example, need to go through a period of shortening before women should do sit-ups. Before that, "the sad thing is, it's exhausting for the mother and you're not really going to see much result from it." She suggests low-key activities such as walking or yoga.

The American College of Obstetricians and Gynecologists says exercise routines "may be resumed gradually as soon as it is physically and medically safe." The time period will vary.

At her six-week postpartum visits, O'Neil stresses the importance of setting aside time to exercise. "Put it into your regimen immediately," she said. "Once you go a year with kids and don't have exercise in the program, it's very hard to get it back in."

New moms feel guilty about devoting time to themselves, but exercise is vital for staying healthy, O'Neil said.

If it's a challenge to find 30 minutes to exercise, do three 10-minute sessions, suggests Sue Fleming, author of "Buff Moms." As she noted, "exercise helps give you the strength, stamina and energy level to keep up with things" — including that baby who will soon be a fast-moving toddler.

Stephanie Dunnewind: sdunnewind@seattletimes.com