WEDNESDAY, Dec. 3, 2014 (HealthDay News) -- Obesity during pregnancy is linked to an increased risk of newborn death, according to a new study, although the researchers found the risk of an infant dying remains very low.
"Even if the risk, relatively speaking, is increased for obese mothers, the actual probability is small," said study lead author Dr. Stefan Johansson, a neonatologist at Karolinska University Hospital in Stockholm, Sweden.
For example, he explained, about 995 of 1,000 Swedish infants survive their first year with the most severely obese mothers, compared to 998 of 1,000 Swedish infants with non-overweight mothers.
Still, the finding has implications, considering current rates of obesity. In the United States, for example, more than one-half of pregnant women are overweight or obese, according to the American College of Obstetricians and Gynecologists.
Johansson's team analyzed more than 1.8 million single live births in Sweden between 1992 and 2010, comparing mothers' body mass index (BMI) in early pregnancy to their newborns' survival. BMI is a ratio of a person's height and weight used to determine how healthy their weight is. A BMI between 18.5 and 24.9 is considered healthy.
The researchers accounted for differences in mothers' age, height, education and number of children, the year they delivered and whether they smoked. The findings were published Dec. 3 in the BMJ.
Infants of mothers who were overweight (BMI between 25 and 29.9) or mildly obese (BMI between 30 and 34.9) were 25 to 37 percent more likely to die than those of non-overweight women, the researchers found.
The rate of infant death among the most obese mothers -- a BMI of at least 40 -- was double the rate among non-overweight mothers, the study authors said. Approximately 58 babies per 10,000 died among the most obese mothers, compared to 24 deaths per 10,000 among non-overweight mothers, the findings showed.
"Overweight and obesity have reached epidemic proportions worldwide, and we all have to realize it's a problem for both individuals and the population at large," said Katrine Mari Owe, a researcher at Oslo University Hospital in Norway.
"It does not mean that overweight or obese women may not have uncomplicated pregnancies and give birth to healthy children. But women of childbearing age should all be aware of the increased risk associated with excess body weight and how it affects the health of the infant," explained Owe, who wrote an accompanying editorial.
This study could not prove that obesity caused any infant deaths, however, and the association between the two is not fully understood but could relate to risk of pregnancy complications, Owe said.
"Maternal overweight and obesity increase the risk of diseases and complications during pregnancy, such as preeclampsia, gestational diabetes, type 2 diabetes, gestational hypertension [high blood pressure] and congenital abnormalities [birth defects]," Owe said, adding these conditions can increase an infant's risk of death.
The most common causes of death among the infants in the study were preterm birth, birth defects and newborn conditions such as birth asphyxia, when a baby doesn't get sufficient oxygen, Johansson said.
Prematurity -- more common in obese pregnancies -- is the number one cause of infant mortality, said Dr. Jennifer Gunter, an obstetrician-gynecologist with Kaiser Permanente in San Francisco. However, other reasons may explain the link between obesity and infant mortality, such as inflammation, so it's important to learn more about the connection, she said.
Meanwhile, women who are already overweight and pregnant can reduce their own babies' risks through various healthy behaviors, Gunter said.
"Control as many variables as you can and get good prenatal care, which reduces the risk of premature delivery," Gunter said. "If you smoke, do your best to quit. Talk with your doctor about the calories per day and weight gain you should be aiming for based on your body weight, and talk with your doctor about the right exercise for you during pregnancy."
Gunter said becoming pregnant often motivates women to make healthy changes if they can learn effective strategies through a discussion with their care provider.
"It's important to frame these discussions in a positive and sensitive way because making people feel bad is both counterproductive and cruel," she said. "Aiming for a BMI below 35 would be the first goal for women who might be struggling with their weight."
It helps to remember that obesity develops gradually over time and that losing some weight is better than losing none, Owe said.
"It is important to focus on a healthy diet," Owe suggested, "and include light physical activity as part of a daily routine to achieve your goal."
The U.S. National Institutes of Health has more about a healthy pregnancy.