Early determinants of first hospital admissions for asthma and acute bronchitis among Swedish children

Aim: To assess the changing impact of prenatal and perinatal factors with increasing age on first admissions for asthma and acute bronchitis. Methods: The Swedish Medical Birth Registry was linked to the National Hospital Discharge Registry for 214276 Swedish children born in 1987 and 1988. The linkage comprised first admissions for acute bronchitis or asthma from 1987 to 1995. First admissions for gastroenteritis were included to distinguish between factors specific to asthma or bronchitis and non-specific factors related to an increased risk of hospital admission. Results: Admissions for asthma and acute bronchitis before 2 y of age had a close dose-response relationship with maternal smoking, low gestational age, low maternal age and older siblings. Very premature children exposed to more than 9 cigarettes a day in utero ran a 10 times higher risk of hospitalization than postmature children without exposure to maternal smoking. However, the proportion of admissions attributed to maternal smoking was only 4.7% after adjustments for significant covariates. The number of significant determinants declined in children with their first admission for asthma after 2 y of age but male gender and low educational level were independent determinants in all age groups. Older siblings were related to an increased risk of first asthma admission before 2 y of age but a slightly reduced risk of first admission after 2 y of age. Conclusion: This study illustrates the varied epidemiology of asthma in different age groups. Maternal smoking was an important determinant for admission before 2 y of age but mainly in combination with other factors. The decline in hospital admissions in early childhood would therefore be small, even after the elimination of maternal smoking, if other significant perinatal and socioeconomic factors were unchanged.