Asthma can lead to serious medical problems, both for mothers-to-be and their fetuses, the program warned. "Simply put, when a pregnant patient has trouble breathing, her fetus also has trouble getting the oxygen it needs," Dr. William Busse, professor of medicine at the University of Wisconsin Medical School and chairman of the panel that developed the guidelines, said in a statement.

Dr. Barbara Alving, the institute's acting director, said "the guidelines review the evidence on asthma medications used by pregnant patients. The evidence is reassuring, and suggests that it is safer to take medications than to have asthma exacerbations."

The guidelines, being published this week in the Journal of Allergy and Clinical Immunology, urge limiting exposure to asthma triggers and treating conditions that can worsen the respiratory ailment, such as allergic rhinitis, sinusitis and acid reflux.

"As important as medications are for controlling asthma, a pregnant woman can reduce how much medication is needed by identifying and avoiding the factors that make her asthma worse, such as tobacco smoke or allergens like dust mites," Dr. Michael Schatz, chief of the Department of Allergy at Kaiser Permanente San Diego Medical Center, said in a statement.

For about 30 per cent of women who have mild asthma when they get pregnant, the illness worsens during pregnancy, according to a recent study by the National Institute of Child Health and Human Development. On the other hand, the study also found that asthma improved in 23 per cent of the women who initially had moderate or severe asthma.

Among the recommendations of the guidelines:

-Albuterol, a short-acting inhaled drug, can be used as a quick-relief medication to treat asthma symptoms. Pregnant women with asthma should have this medication available at all times.

-For women with persistent asthma, inhaled corticosteroids are the preferred medication to control the underlying inflammation.

-For women whose persistent asthma is not well-controlled on low doses of inhaled corticosteroids alone, the guidelines recommend either increasing the dose or adding another medication - called a long-acting beta agonist.