Approximately 3 to 8 percent of pregnant women have asthma, but the impact of the disease or its treatment on pregnancy outcomes is not clearly understood. Bracken and colleagues studied more than 800 pregnant women with asthma to determine the effects of asthma and its treatment on pregnancy outcomes.

They screened all pregnant women attending 71 clinics in New England before 24 weeks of gestation. Women with a history of physician-diagnosed asthma were invited to participate in the study. The 872 women who agreed were matched with 1,333 control patients. All participants were interviewed at 20, 28, and 36 weeks of gestation, and again in the hospital shortly after delivery. The data collected included demographic information, symptoms, and the use of health services, in addition to pregnancy outcomes. Mothers with asthma were classified into four groups based on severity of symptoms (i.e., intermittent, mild-persistent, moderate-persistent, and severe-persistent). An overall asthma severity score was calculated for each woman based on symptoms and medications used. This score was related to gestational age and weight at delivery.

In the initial results, preterm delivery was associated with asthma. The degree of risk was not related to the severity of asthma but was related to the use of chromone derivatives (such as cromolyn), theophylline, and oral steroids. When confounding variables were controlled in multivariate analysis, preterm delivery was not associated with asthma, but gestation was reduced by 2.22 weeks in women using oral steroids and by 1.11 weeks in those using theophylline. Conversely, fetal growth restriction was associated with asthma of moderate-persistent severity.

The authors conclude that more severe forms of asthma are associated with intrauterine growth restriction, possibly caused by fetal hypoxia. These findings validate recommendations to treat pregnant asthmatic patients with beta agonists. Conversely, preterm delivery appears to be related to treatment with oral steroids or theophylline rather than to the severity of asthma. A surprising finding was that one third of women without asthma in the control group reported symptoms consistent with asthma. The authors call for greater vigilance to detect unrecognized asthma in pregnant patients.