"How much weight should I gain during pregnancy?" is one of the questions that torments expectant moms, and doubly so because the answers offered up by one's mother, mother-in-law, friends and physicians are often all over the map.
But new guidelines, published yesterday, say the science is pretty clear: A woman should gain 25 to 35 pounds (11 to 16 kilograms) during pregnancy if she is a healthy weight prior to conception.
At the same time, the blue-ribbon panel of experts says that the growing legion of women who are overweight or obese when they get pregnant should gain significantly less - as little as 11 lbs. (5 kgs.) during the nine months of gestation.
"Good pregnancy outcomes are associated with healthy weight of the mother," said Kathleen Rasmussen, a professor of nutrition at Cornell University in Ithaca, N.Y., and chair of the guidelines committee.
She said that, currently in the U.S., between 43 and 70 per cent of women gain more weight during pregnancy than recommended in the guidelines. The problem is more acute among expectant moms who overweight or obese.
"More women need to have a normal BMI (body mass index - an approximation of fat) before conception," Dr. Rasmussen said.
About two in three women of childbearing age in the U.S. are overweight or obese - meaning they have a BMI of 25 or more. In Canada, the rate is almost three in five, only slightly lower.
The new guidelines, issued by the U.S. Institute of Medicine, warn that when women are overweight or when they gain too much weight during pregnancy, it can result in a host of serious health problems for both mother and child.
In particular, pregnant women with excess weight are at high risk of developing gestational diabetes and pre-eclampsia (a condition characterized by high blood pressure and fluid retention), and they have much higher rates of miscarriage and cesarean delivery.
The babies of overweight and obese women are more likely to be of low birth weight which, in turn, can result in a wide range of health concerns, from breathing problems to developmental delays.
The new guidelines, which stretch out to 250 pages, stress, above all, that women should be getting a lot more counselling about nutrition and fitness prior to conception and it should continue throughout pregnancy and post-partum. That is because women who put on too much weight when they are carrying a child are less likely to lose it after the birth.
"We need to change the culture surrounding pregnancy and weight gain," said Anna Maria Siega-Riz, an associate professor in the department of maternal and child health at the University of North Caroline in Chapel Hill, N.C.
Donald Davis, past president of the Society of Obstetricians and Gynaecologists of Canada and a practitioner in Medicine Hat, Alta., praised the guidelines for their thoroughness and clarity.
But he said that one of the frustrations for physicians who counsel their patients about the importance of healthy weight gain is that the "message doesn't always get through."
Dr. Davis said that there is indeed a widespread cultural belief that weight gain isn't that important. "My patients say: 'My Mom gained a lot of weight, the baby was fine, and she lost the weight.'"
The key distinction, he said, is that far more women today are overweight before they get pregnant.
The new guidelines note that, in addition to women being heavier before pregnancy, they also tend to be older, often have underlying health conditions such as diabetes and hypertension that are exacerbated by pregnancy, and are far more likely to have multiple births.
While the new guidelines lay out different weight gain targets based on a woman's pre-pregnancy weight, they have done away with a number of other distinctions.
For example, there are no longer distinct weight gain recommendations for women of various ethnic and racial backgrounds, nor for adolescents.
While the guidelines focus on singletons, there are also weight gain recommendations for women carrying twins. Those who are of normal weight should put on 37 to 54 lbs., overweight expectant moms of twins should gain 31 to 50 lbs., and those who are obese should limit their weight gain to 25 to 42 lbs.
There are no guidelines for mothers of triplets through octuplets because, according to the research, there is insufficient data to show what is healthy.