TUESDAY, June 11, 2013 (HealthDay News) -- Overweight or obese women who are pregnant are more likely to give birth prematurely, and the risk of preterm delivery increases with their amount of excess weight, according to a study of more than 1.5 million deliveries in Sweden.
Researchers speculate that the health problems associated with overweight and obesity -- high blood pressure, insulin resistance, high cholesterol, increased chance of infection -- have a direct impact on a woman's ability to carry their child to term, according to the study, which was published in the June 12 issue of the Journal of the American Medical Association.
Birth of an infant prior to 37 weeks of gestation is the leading cause of infant mortality, neonatal illness and long-term disability in children, said researchers led by Dr. Sven Cnattingius of the Karolinska Institute in Stockholm.
A U.S. expert not involved with the study said he wasn't surprised by the findings.
"This just reinforces the fact that the complications of obesity and additional weight gain are deleterious to both mother and fetus," said Dr. Raul Artal, a professor and chairman of the department of obstetrics, gynecology and women's health at the Saint Louis University School of Medicine.
Artal said this study, along with previous research, makes the case that overweight and obese women who are pregnant can protect the health of their unborn child by maintaining their current weight or even shedding some pounds.
"The concept that we propagated for years that pregnancy is not a good time for weight loss and physical activity is wrong," he said.
For the study, researchers analyzed the records of 1.59 million births between 1992 and 2010, reviewing the body-mass index (BMI) of the women at their first prenatal doctor's visit as well as information recorded following birth about health risks, maternal diseases and pregnancy complications.
BMI is a measure of body fat based on height and weight.
The statistics came from the Swedish Medical Birth Register, which maintains detailed data on all births in that nation and serves as a valuable resource for researchers, a U.S. expert said.
"There's a lot of good obstetric data that comes out of Sweden because of that register," said Dr. Russ Fothergill, vice chairman of the department of obstetrics and gynecology at Scott and White Healthcare in Temple, Texas.
The researchers found that the risk of extremely premature (22 to 27 weeks), very premature (28 to 31 weeks) and moderately premature (32 to 36 weeks) deliveries increased with a woman's BMI.
The authors said that even though the study is Swedish, the results can be generalized to other populations with similar or higher rates of maternal obesity or preterm delivery.
For example, the United States has preterm delivery rates twice as high as Sweden's. In the United States, slightly more than half of women are either overweight or obese in early pregnancy, and severe obesity is much more common than in Sweden. Extremely preterm births accounted for 25 percent of all U.S. infant deaths among single births, and extremely preterm birth is also the leading cause of long-term disability, according to study background information.
The study's results seem to suggest an interplay between health risks linked to overweight and obesity, said Dr. Jill Rabin, chief of ambulatory care, obstetrics and gynecology, and head of urogynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y.
Earlier studies have found that women who suffer a spontaneous preterm delivery have increased levels of inflammatory proteins called cytokines, which are associated with neonatal health problems such as weakening of the membranes and preterm contractions.
Both obesity itself and the health problems that come with obesity increase the amount of inflammation in a woman's body, Rabin said. Obesity and the inflammation that accompanies it also make it more likely that a woman could suffer an intrauterine bacterial infection.
"Obese women have increased risk of urinary tract infections and vaginal infections," Rabin said. "These increase the risk of preterm delivery."
The study noted that a bacterial infection is considered the most important risk factor for spontaneous extremely preterm delivery.
The American Congress of Obstetricians and Gynecologists has issued a recommendation that it is OK for women who are overweight or obese to not gain weight during pregnancy, or even to lose weight, said Artal, of Saint Louis University.
"I consider pregnancy to be an ideal time for behavioral modification," Artal said. He noted that pregnant women have an improved chance to adopt healthy habits because they have better access to medical care than at any other time in their life, are more prone to comply with recommendations because they have the added responsibility of their unborn child, and are under close medical supervision.
Although the study found an association between maternal obesity and premature birth, it did not establish a cause-and-effect relationship.
For his part, Fothergill said the study cries out for follow-up research that will better elaborate upon the link between obesity and premature birth.
"There needs to some additional research that explains why these women are delivering early," he said. "What is it about obesity that's making these women go into labor early?"
The March of Dimes has more about overweight and obesity during pregnancy.