Women with high glucose readings early in pregnancy are at increased risk of having a baby with heart defects, even if they do not have full-blown diabetes, a new study found. Diabetes during pregnancy is a known risk factor for heart problems in babies.

Researchers studied data on 19,107 mothers, members of two large health care systems, of whom 811 gave birth to babies with congenital heart disease. The data included blood glucose measurements done between four weeks before conception and the 14th week of gestation.

The heart develops very early in pregnancy. The fetal heart begins to beat at about three weeks after conception, and by the eighth week of gestation it has clearly developed chambers.

The study, in The Journal of Pediatrics, found that for each 10 milligrams per deciliter increase in plasma glucose, there was an 8 percent increase in the risk for giving birth to a baby with heart defects, even after adjusting for maternal age, pre-pregnancy body mass index, and the presence of pre-existing maternal diabetes.

“This is important because it is a modifiable risk factor,” said the lead author, Dr. Emmi I.T. Helle, a pediatrician at Children’s Hospital in Helsinki. “A mother should exercise during early pregnancy — an hour of walking every day is enough — because we know that exercise improves insulin sensitivity. Maintaining a healthy diet is also very important.”