Prematurity as a Predictor of Childhood Asthma among Low-Income Children
Introduction
Asthma is a serious chronic illness affecting more than 6 million children in the United States (1). The health services burden of asthma is substantial, accounting for more than 700,000 emergency department (ED) visits and nearly 200,000 hospitalizations among children annually (2). Over the past two decades the prevalence of asthma among children in the United States has increased, especially among minority populations (3), although the causes of this increase remain unclear (4). Asthma prevalence and health services use are disproportionately high for non-Hispanic black children compared with white children, including higher rates of ED use, hospitalizations, and mortality 2, 5, 6, 7, 8, 9, 10.
The underlying root of these differences appears to be associated with multiple factors including lower socioeconomic status 8, 11, urban residence 8, 12 and a range of related environmental characteristics 11, 13, 14. In addition, there is increasing evidence suggesting that improved survival rates among premature infants may have long-lasting implications in terms of reduced lung function. Birth characteristics such as low birth weight (LBW) 9, 10, 15, 16, 17, 18, 19 and prematurity 18, 19, 20, 21, 22, 23, 24 have been associated with asthma later in life. At the same time, substantial differences in birth weight and prematurity exist between race and ethnic groups. For example, black infants have rates of prematurity and very LBW that are two- to three-fold that of white infants 25, 26. Although birth weight is closely related to gestational age, substantial classification differences may occur when prematurity risk categories are established strictly on the basis of birth weight rather than gestational age. For example, fewer than 70% of infants who are LBW (<2,500g) are also considered preterm (<37 weeks' gestation); conversely, half of preterm infants are also considered LBW (27). Some infants classified as LBW may have birth weights regarded as being appropriate for gestational age (AGA) when their shorter duration of gestation is taken into account, whereas another infant born after the same duration of gestation may have a sufficiently LBW to be considered small for gestational age (SGA). An infant's size for gestational age may have different effects on their risk of developing asthma later in life, although few studies have taken this factor into consideration 24, 28.
Given the elevated prevalence of asthma among racial minorities and the reported associations among birth weight, prematurity, and race, we wished to better understand the risks of childhood asthma, taking these potentially confounding effects into account. Importantly, the distinctions between birth weight, gestational age, and size for gestational age may offer additional insight into understanding the disproportionate prevalence of asthma across race and ethnic groups, such as the higher prevalence of asthma among blacks 9, 16. The objectives of this study were to evaluate the association between gestational age and the prevalence of asthma later in childhood among low-income children and to assess the degree to which this association may differ between black and white children.
Section snippets
Methods
This study is based on an analysis of administrative claims data from the Michigan Medicaid program and was approved by the University of Michigan and Michigan Department of Community Health (MDCH) Institutional Review Boards.
Results
Overall, birth certificate data were successfully matched for 160,561 children (81% of the initial sample); missing birth certificate information was somewhat higher among children 15–18 years old (2%), compared with those in the younger age cohorts (1%). Of these children for whom birth certificate information was successfully matched with Medicaid records, we excluded 2,317 cases (1%) with missing or extreme values for birth weight or gestational age. Among the remaining 158,244 children,
Discussion
Our findings indicate that prematurity and birth weight are associated with the risk of persistent asthma later in childhood and that these birth characteristics mediate racial differences in asthma prevalence among Medicaid beneficiaries. Consistent with previous reports, our findings indicate that children born with LBW 9, 10, 15, 16, 17, 18, 19 and those born preterm 18, 19, 20, 21, 22, 23, 24 are at an increased risk of developing asthma later in childhood. In contrast to prior studies 24,
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This work was supported by the Michigan Department of Community Health and the Blue Cross Blue Shield of Michigan Foundation. We thank Glenn Copeland of the Michigan Department of Community Health for his assistance with birth record matching and Melissa McPheeters, PhD, for her input on our initial manuscript drafts.