Although pregnancy is purported to affect maternal asthma, the literature has not been reviewed systematically. The purpose of this systematic review was to determine, among pregnant women with asthma, whether pregnancy and stage of pregnancy influence maternal asthma severity. Six electronic databases were searched in January 2003 for prospective studies of currently asthmatic, pregnant women who were enrolled before the third trimester and assessed with objective measures of asthma severity or validated severity scales. Three studies reported 54 pregnant women with asthma who met the inclusion criteria. The most valid study indicated that bronchial hyperresponsiveness to methacholine improved between preconception and pregnancy in 69% of the women, although it deteriorated in 31% of the women. Further, this improvement peaked in the second trimester, reverted after delivery, and was greatest among those women who were most hyperresponsive initially. Lung function also showed a trend towards improvement during pregnancy that was not significant. Large, well-conducted population-based studies that explore different aspects of asthma severity are needed to substantiate these preliminary results.