Asthma is a common chronic disease affecting the lives of reproductive age women and is associated with 8-13% of pregnancies [1]. While maternal asthma has been consistently associated with significant perinatal morbidities and mortality [2, 3], impacts on fertility are conflicting. In light of limited and conflicting evidence, the aim of this study was to examine the impact of asthma and asthma medication use on fecundability and time to pregnancy. Participants were healthy, nulliparous women recruited to the Screening for Pregnancy Endpoints (SCOPE) study between November 2004 and February 2011 in Auckland (New Zealand), Adelaide (Australia), Cork (Ireland), and Manchester and London (United Kingdom). The SCOPE study is a multicenter prospective cohort study with the primary aim of developing screening tests for prediction of pre-eclampsia, spontaneous preterm birth and small-for-gestational-age neonates. Ethical approval was obtained from local ethics committees (New Zealand AKX/02/00/364, Australia REC 1712/5/2008, London, Leeds and Manchester 06/MRE01/98 and Cork ECM5 (10) 05/02/08). All women provided written informed consent. Detailed methods have been described previously [4]. Asthma was self-reported and identified according to the question "Have you been diagnosed with asthma by a doctor?". Asthmatic women were further divided by asthma symptoms and asthma medication use and then classified as having former asthma (doctor diagnosed asthma, no symptoms in the previous 12 months and no use of asthma medications) or current asthma (doctor diagnosed asthma, symptoms in the previous 12 months or use of asthma reliever or preventer medications). Women with current asthma were further divided according to use of intermittent reliever medications only (i.e. short-acting β-agonists (SABA)) or additional use of reliever medications (i.e. inhaled corticosteroids (ICS) with or without long-acting β-agonists (LABA)). This resulted in three asthmatic subgroups: former asthma, SABA and ICS±LABA. Information was collected on demographics, smoking, family, medical and gynaecological history, and anthropometry (height and weight). Maternal ethnicity was self-reported and categorised as Caucasian or other. The socioeconomic index is a measure of socioeconomic position according to income and education, corrected for age [5]. This index was calculated for all women in the SCOPE study (range 10-90), with a higher score indicating higher socioeconomic status [6]. Self-reported polycystic ovary syndrome was categorised as yes (confirmed by a scan and/or a blood test) or no/unsure. Cigarette use in the 3 months pre-pregnancy was coded as any versus not smoking. Age at menarche was reported as a continuous variable. Information collected from the biological father included age and BMI. Self-reported time to pregnancy (TTP) was defined as the duration of sex (in months) without contraception before the current pregnancy and was collected in the first trimester. Subfertility was defined as TTP >12 months. Fecundability odds ratio...