"Diabetes is the tail end of a spectrum of metabolic abnormalities," said study lead author Dr James Priest, a postdoctoral scholar in paediatric cardiology at Stanford University in California. "We already knew that women with diabetes were at significantly increased risk for having children with congenital heart disease.
Blue baby syndrome
"What we now know... is that women who have elevated glucose or blood sugar values during pregnancy that don't meet our diagnostic criteria for diabetes also face an increased risk."
The researchers examined blood samples taken from 277 California women during the second trimester of pregnancy.
The study participants included a control group of 180 women who had babies without heart defects. The other women had babies with one of two serious heart defects.
Fifty-five babies were born with structural problems in the heart and the blood vessels that connect the heart to the lungs, called tetralogy of Fallot. It is one of the heart defects that cause blue baby syndrome, in which a baby gets too little oxygen, the researchers said.
The other 42 babies were born with dextrotransposition of the great arteries, in which the two main arteries leading from the heart are switched in position. This prevents oxygenated blood from the lungs from circulating to the body, the researchers explained.
New avenues for research
The researchers' analysis linked elevated blood sugar levels even if below the cutoff for diabetes with an increased risk of tetralogy of Fallot, but not with dextrotransposition of the great arteries.
Also, the researchers found no significant association between levels of insulin the hormone that regulates blood sugar and either type of heart defect.
"I'm excited by this research," Priest said in a university news release. "Most of the time we don't have any idea what causes a baby's heart defect."
Several other kinds of structural birth defects, in addition to heart defects, have been linked with diabetes, he added.
The new findings could lead to new avenues for research, added study senior author Gary Shaw.
"This new work will motivate us to ask if underlying associations with moderately increased glucose levels may be similarly implicated in risks of some of these other birth defects," Shaw, a professor of paediatrics in neonatal and developmental medicine, said in the news release.
Image: Baby with a heart monitor from iStock