Study population
The study population consisted of 136 children from three prospective cohorts(9, 15, 16) of very preterm infants (gestational age <32 weeks), one also including 56 term born controls (see Table 1 and online supplement for details). Antenatal and neonatal data were collected prospectively from the patient records. There were 85 children with and 51 without a previous diagnosis of bronchopulmonary dysplasia (BPD), defined as being born at < 32 weeks gestation and still being treated with extra oxygen at 36 weeks postmenstrual age. A history of respiratory symptoms, medication and allergy was obtained from a questionnaire completed by the accompanying parent/caregiver at the 12-year visit.