TUESDAY, March 15, 2016 (HealthDay News) -- Being overweight or obese during pregnancy may mean higher weights in newborns, a new study suggests.
Researchers have long known that heavier moms-to-be tend to have bigger babies. But it hasn't been clear that the extra pounds, per se, are the reason.
So the new study, reported March 15 in the Journal of the American Medical Association, dove into the genetics of the issue.
Using genetic information from more than 30,000 women, researchers found that women who carried more obesity-related gene variants tended to give birth to bigger babies.
The same was true when the researchers considered mothers' genetic vulnerability toward having high blood sugar -- a precursor to type 2 diabetes. On the other hand, women with many gene variants tied to high blood pressure tended to have smaller newborns.
The findings offer good evidence that heavier weight and higher blood sugar in moms directly cause higher birth weight, according to Rachel Freathy, one of the researchers on the study. She's a fellow at the University of Exeter Medical School, in the United Kingdom.
"Genetic association is stronger evidence of cause-effect than just measuring body mass index, glucose [blood sugar] or blood pressure," Freathy explained.
That's because a woman's weight is influenced by education, diet, smoking and other lifestyle factors, Freathy said -- and those factors also affect her baby's birth weight.
"But a genetic 'score' for blood pressure, [obesity] or glucose is not influenced by these lifestyle factors," she said.
The findings are based on genetic data from women who took part in 18 studies conducted in Europe, North America and Australia.
Freathy's team assigned each woman "genetic scores" for obesity [before pregnancy], blood sugar, blood pressure, and cholesterol and triglyceride levels. So, a woman who carried a large number of genes related to obesity, for example, would have a higher genetic score for that trait.
Overall, the study found, babies' birth weight rose in tandem with mothers' genetic scores for obesity and blood sugar.
For example, each standard deviation in the obesity score -- about a 4-point increase in a woman's body mass index (BMI) -- was related to an extra 2 ounces in a newborn's birth weight, the study said.
Body mass index is a rough estimate of a person's body fat based on height and weight measurements. A BMI of 18.5 to 24.9 is considered normal, according to the U.S. Centers for Disease Control and Prevention.
The researchers also found that for every 10-point increase in a woman's systolic blood pressure meant a 7-ounce drop in her baby's birth weight. Systolic is the top number in a blood pressure reading.
It makes sense that higher blood pressure scores would be related to lower birth weight, said Dr. Jennifer Wu. She's an obstetrician at Lenox Hill Hospital, in New York City.
That's because high blood pressure appears to restrict fetal growth, explained Wu. She wasn't involved in the current research.
Wu agreed that the genetic associations in this study strengthen the case that a pregnant woman's weight, blood sugar and blood pressure all directly affect her newborn's weight.
The findings also support what's already recommended for a healthy pregnancy, Wu said.
"It's very important that women go into pregnancy at a healthy weight," she said. Women who are obese, or show spikes in blood pressure or blood sugar during pregnancy, will likely need extra monitoring, Wu added.
"We want babies to be a healthy weight, not too heavy and not too light," Freathy said.
The researchers said they plan to tackle the next big question: Do the effects of mothers' weight, blood sugar and blood pressure on newborns' weight eventually translate into health effects later in life?
For now, Freathy agreed that the findings underscore the importance of maintaining a healthy weight through diet and exercise -- before and during pregnancy.
She also stressed that pregnant women should have regular prenatal care, including routine blood pressure and blood sugar checks.
The U.S. National Institutes of Health has advice on having a healthy pregnancy.