Previous maternal use of the oral contraceptive pill (OCP) has been linked with asthma in subsequent offspring and has been implicated in the increased prevalence of childhood asthma in recent decades. We conducted a matched case–control study to test the hypothesis that maternal OCP used close to conception is associated with asthma in the offspring, particularly in children with coexistent eczema. We examined maternal OCP exposure in relation to asthma in the offspring (n = 6730) compared with offspring with no asthma (n = 6730) further stratifying by eczema, age group, treatment category and gender of the offspring. Maternal use of OCP was classified as: no OCP use in the 2 years prior to conception; past OCP use within 2 years but >6 months before conception; and recent OCP use within 6 months of conception.
The adjusted odds ratio (OR) for asthma in the offspring was 1.16 [95% confidence interval 1.06, 1.27] among mothers who were recent users of the OCP when compared with mothers who had not used the OCP. Past OCP use was not associated with asthma in the offspring. In the stratified analyses, we observed weak but statistically significant associations between recent maternal OCP use and asthma in the offspring among children: without a history of eczema (adjusted OR 1.22 [1.09, 1.36]), those aged ≤3 years (adjusted OR 1.24 [1.12, 1.37]), those not on treatment for their asthma (adjusted OR 1.33 [1.12, 1.58]) and among females (adjusted OR 1.34 [1.13, 1.51]). We did not find convincing evidence for a causal relationship between maternal OCP used close to conception and asthma in the offspring. The small statistically significant associations were not among children with characteristic features of asthma such as those with eczema and may be due to bias, uncontrolled confounding or chance.