Asthma flare-ups early in pregnancy may raise the risk of birth defects, a new study suggests - highlighting, researchers say, the importance of good asthma control in pregnant women.

In a study of 3,477 asthmatic women who gave birth at least once between 1990 and 2000, Canadian researchers found that those who’d had symptom flare-ups in the first trimester were 48 percent more likely to have a baby with a congenital malformation.

Overall, 13 percent of babies born to women with asthma attacks had at least one malformation, such as a birth defect of the heart, facial structures, spine or digestive system. That compared with 9 percent of infants whose mothers had well-controlled asthma in early pregnancy.

The findings are published in the Journal of Allergy & Clinical Immunology.

Poorly controlled asthma during pregnancy may be dangerous for the fetus because it can hinder the flow of blood and oxygen to the placenta.

Because the first trimester is a critical period in which most birth defects take shape, asthma attacks early in pregnancy may be particularly concerning, according to Dr. Lucie Blais, the lead researcher on the new study.

“Women who are planning to become pregnant should make sure their asthma is under control,” said Blais, of the University of Montreal.

“If it’s not well-controlled, they should go to their doctor,” she told Reuters Health.

Some women, as well as doctors, may be concerned about the potential effects of asthma medications during pregnancy, Blais noted. One type of drug has, in fact, been linked to birth defects: oral corticosteroids, which are used to treat acute asthma attacks.

But that, Blais said, makes an even stronger case for good asthma control to prevent flare-ups - which is usually achieved by taking inhaled corticosteroids.

“This study highlights the need for women to take controller medication to avoid exacerbations and to avoid using oral corticosteroids,” Blais said.

SOURCE: Journal of Allergy & Clinical Immunology, June 2008.