NEW YORK (Reuters Health) – Pregnant women with asthma are at increased risk of having babies of low birth weight and of premature deliveries, which largely accounts for the high perinatal mortality rates in this population, according to a Canadian study involving more than 41,000 singleton pregnancies.

Although most others studies on this topic have indicated no increased risk of adverse pregnancy outcomes, small sample sizes limited their statistical power, Dr. Lucia Blaise, at the University of Montreal, and colleagues note in the February issue of Thorax.

To better quantify the risk associated with asthma in pregnancy, the research team combined three health administrative databases from Quebec. Their cohort included 13,100 women with asthma and a 33% random sample (n = 28,042) of women without asthma who had at least one delivery between 1990 and 2002.

The crude odds ratio of perinatal mortality associated with maternal asthma was 1.35, the report indicates.

Compared with controls, women with asthma were more likely to deliver infants at < 37 weeks gestation (10.4% vs 6.7%), birth weight no greater than 2500 g (9.2% vs 5.7%), and small for gestational age (14.4% vs 10.5%).

Low birth weight and preterm delivery were very strong predictors of perinatal mortality, the authors note. By contrast, among normal weight and term babies, maternal asthma did not increase the risk of perinatal death, suggesting that