Pregnant women need to keep their asthma under control, because if they cannot breathe properly, neither can the unborn baby, according to new U.S. guidelines issued on Tuesday.

Although asthma drugs may affect the developing baby, they pose less of a risk than the breathing problems caused by asthma, the panel of experts who issued the guidelines said.

“The evidence is reassuring, and suggests that it is safer to take medications than to have asthma exacerbations,” said Dr. Barbara Alving, acting director of the National Heart, Lung, and Blood Institute, which directed the panel.

“The guidelines should be a useful tool for physicians to develop optimal asthma management plans for pregnant women.”

Asthma affects more 20 million Americans and is one of the most common potentially serious medical conditions to complicate pregnancy. The panel estimated it affects up to 8 percent of pregnant women, who may be afraid to take drugs to control their condition.

“Simply put, when a pregnant patient has trouble breathing, her fetus also has trouble getting the oxygen it needs,” said Dr. William Busse of the University of Wisconsin Medical School, who chaired the National Asthma Education and Prevention Program.

Asthma is associated with increased risk of infant death, preeclampsia - a serious condition marked by high blood pressure - premature birth, and low birth weight.

And in 30 percent of women who have mild asthma at the beginning of their pregnancy, the asthma worsens.

The guidelines say that albuterol, an inhaled drug, should be used for quick relief and kept on hand.

Women with asthma symptoms at least two days a week or two nights a month have persistent asthma and need daily medication. Inhaled corticosteroids, notably budesonide, are the preferred drugs, the panel said.

Doses should be increased if the drugs do not control symptoms, or other classes of drugs can be added. Oral corticosteroids may be needed for the most severe cases, despite confusing information on their safety.

The guidelines, published in the January issue of the Journal of Allergy & Clinical Immunology, say obstetricians should be part of a pregnant patient’s asthma treatment team.