High birth weight is tied to obesity and diseases such as cancer and asthma later in life, researchers say.High birth weight is tied to obesity and diseases such as cancer and asthma later in life, researchers say. (CBC)

Women who gain large amounts of weight during pregnancy tend to have heavier babies who are at higher risk for obesity as adults, researchers say.

Until now, researchers hadn't shown whether the tendency results from the weight gain itself or other factors such as genetics that the mother and baby share.

The study in Thursday's online issue of the medical journal The Lancet focused on more than 513,000 American women. To eliminate genetic causes of high birth weight, the researchers compared successive babies born to the same mothers between 1989 to 2003.

"Since high birth weight, in turn, increases risk for obesity and diseases such as cancer and asthma later in life, these findings have important implications to general public health," said study co-author Dr. David Ludwig, director of the Optimal Weight for Life Program at Children's Hospital Boston.

Newborns of women who gained more than 53 pounds during pregnancy were nearly 150 grams or 5.2 ounces heavier than infants of women who gained 22 pounds or less, the researchers found.

High-birth-weight babies — 8.8 lbs or more — accounted for 12 per cent of all births.

But the study cannot conclusively resolve the chicken-and-egg issue of whether those babies' high birth mass resulted from, or was a cause of, their pregnant mothers' weight gain.

Obesity prevention before birth

The correspondence between a mother's pregnancy weight gain and a baby's birth weight held after excluding women who ever smoked or delivered by caesarean section, and those whose pregnancies lasted fewer than 39 weeks or more than 40 weeks.

"It's never too early to start preventing obesity," said Stephan Rossner, a professor in the obesity unit at Karolinska Hospital in Sweden who was not connected to the study.

"It may be uncomfortable for mothers to eat less and change their lifestyle, but after nine months they will get a great payoff for their children."

In 2009, Health Canada formally updated its guidelines on pregnancy weight gain, which include:

  • Women with a body mass index or BMI of less than 18.5 prior to pregnancy, which is considered underweight: recommended range of total weight gain is between 28 and 40 pounds.
  • Women in the normal weight category with a BMI between 18.5 and 24.9: recommended range of total weight gain from 25 to 35 pounds.
  • Overweight women with a pre-pregnancy BMI between 25 and 29.9: total recommended weight gain between 15 and 25 pounds.
  • Obese women with a pre-pregnancy BMI of greater than 30: advised to gain no more than 11 to 20 pounds during pregnancy.

Pregnant women who gain too much weight may suffer from complications like diabetes and high blood pressure. But less is known about the effects of the extra pounds for babies, aside from a higher risk of needing a C-section.

Obesity prevention during pregnancy should encourage pregnant women to improve their diet and exercise rather than trimming down, said Neal Halfon, of the Center for Healthier Children, Families and Communities at the University of California.

Women who are a healthy weight are generally advised to gain a couple of kilograms in the first trimester, and then, on average, about 0.5 kg (1 pound) per week in the second and third trimesters.

Animal studies suggest that excess weight gain during pregnancy affects the uterine environment, leading to changes in the baby's hypothalamus, pancreatic islet cells, fat tissue and other systems that regulate body weight.

The study was funded by the U.S. National Institutes of Health.

With files from The Associated Press