Gestational age, birth weight and childhood hospitalizations for asthma and other wheezing disorders: evidence from the "Children of 1997" birth cohort

Abstract

Background: In developed Western settings, preterm birth (<37 weeks gestation), early term birth (37 to <39 weeks gestation) and lower birth weight are associated with higher risk of childhood asthma, but such observations may be confounded by socioeconomic position (SEP). We assessed the associations of gestational age and birth weight with childhood hospitalizations for asthma and other wheezing disorders in a developed non-Western setting with little clear social patterning of preterm birth or birth weight. Methods: Using Cox regression, we examined the adjusted associations of gestational age and birth weight with public hospital admissions for asthma, bronchitis and bronchiolitis (International Classification of Diseases, Ninth Version Clinical Modification 466, 490 and 493) from 9 days to 12 years of age in a population-representative birth cohort “Children of 1997” of 8,327 Hong Kong Chinese born in April and May 1997. Analyses were adjusted for sex, birth hospital, mode of delivery, birth order, mother’s age, mother’s place of birth, early life secondhand smoke exposure, parental history of allergies and SEP (highest parental education). Results: We found higher risk of hospitalizations in children who were born at late preterm (34 to <37 weeks) [hazard ratio (HR) 1.98, 95% confidence interval (CI) 1.47 to 2.66], compared to children born at full term (39 to <41 weeks). Children born at early term (37 to <39 weeks) did not have higher risk of hospitalizations (HR 1.02, 95% CI 0.85 to 1.22), and nor did those born at late term (41 to <42 weeks) (HR 0.77, 95% CI 0.59 to 1.01). However, post-term births (≥42 weeks) had lower risk of hospitalizations (HR 0.56, 95% CI 0.32 to 0.98). Birth weight-for-gestational age z-score was not associated with hospitalizations (HR 0.95, 95%CI 0.88 to 1.03). Conclusion: The association of preterm birth with asthma and other wheezing disorders is likely to be biologically mediated, being consistent across settings and persistent beyond early childhood. However, whether early term birth or lower birth weight leads to higher risk of childhood wheezing disorders remains unclear. Further studies should focus on clarifying the underlying biological mechanisms in order to identify potential preventive interventions for asthma and other wheezing disorders in early life.