Asthma is probably the most common potentially serious medical problem to complicate pregnancy. In recent national surveys in the USA, 8.4–8.8% of pregnant women reported current asthma.1 Since 1970, many published articles have suggested that women with asthma experience more complications of pregnancy than women who do not have asthma. The most commonly reported increased risks have been for pre-eclampsia,2^–12 preterm birth2 5 6 8 9 13^–15 and infants with low birth weight or intrauterine growth restriction.2 5 6 8–10 12 15 16The most severe complication of pregnancy from the infant standpoint is fetal or neonatal death. Although one study from the USA in 1970 reported a significant 80% increased risk of perinatal mortality in infants of women with asthma compared with those without asthma,17 and another study from Sweden in 1972 reported a more than doubling of neonatal mortality in infants of mothers with asthma,2 11 studies published between 1988 and 2007 did not demonstrate a significant increased risk of fetal mortality, neonatal mortality, or both (perinatal mortality) in the infants of women with asthma compared with women without asthma.3 7 10 15 16 18^–23 Since the prevalence of perinatal mortality is fortunately low, a type 2 error due to low statistical power could explain many of these negative results. Indeed, sample sizes for women with asthma in 7 of the 11 negative studies were less than 2000.3 10 16 18–20 22 Moreover, a significant 21% increased risk of perinatal mortality was reported in the second largest cohort of women with asthma studied to date (n = 36 965).6

Against this background, the study by Breton et al24 in the current issue of Thorax (see page 101) of a database cohort …