First trimester fetal size and prescribed asthma medication at 15 years of age.

There is increasing evidence that antenatal factors predispose to childhood asthma. We tested the hypothesis that reduced first trimester fetal size is associated with increased risk for asthma at 15 years of age.Fetal size in the first and second trimester was ascertained by ultrasound scan. The primary outcome of being dispensed one or more asthma medications by the family doctor in the year before the 15th birthday was determined from routinely acquired dispensing data.Dispensing data were available for 1699 (88% of the original cohort) participants at 15 years of age and questionnaire data for 750 (39%). Each reduction in z-score for first trimester size was associated with increased odds for dispensed asthma medication at 15 years of age (OR 1.26, 95% CI 1.03-1.54) and self-reported use of asthma medications (OR 1.55, 95% CI 1.16-2.08). Overall, first and second trimester size and forced expiratory volume in 1 s at ages 5, 10 and 15 years were reduced for those dispensed asthma medications compared with those not dispensed asthma medications (p=0.003).Antenatal factors that are active by the first trimester may contribute to respiratory well-being throughout childhood. Dropout from a birth cohort study can overestimate of the magnitude of any true association.