Link Between Preterm Birth and Asthma Risk Reexamined

Maternal smoking may be more likely than birth age to cause asthma in kids.

The increased risk for asthma developing in children born in late preterm at 34 to 36 weeks could have less to do with birth age than with exposure to maternal smoking, according to a new birth cohort study.

"Although results of many studies have indicated an increased risk of asthma in former late preterm infants, most of these studies did not fully address covariate imbalance," explained the study’s lead author, Gretchen Voge MD, Division of Neonatology, Children's Hospitals and Clinics of Minnesota (pictured). "We assessed whether known risk factors for asthma (e.g. confounders) account for the association between late preterm and the risk of asthma."

Investigators compared the course and risk factors of 282 children born in late preterm to 297 born at full term, matched for demographics like sex, birth hospital, and size for gestational age. The cohort was identified from among 8,000 births in one Minnesota county from 2002 to 2006, tracked by the Rochester Epidemiology Project.

The cumulative incidence of asthma at 5 years of age in those born late preterm was 29.9%, compared to 19.5% of those born at full term. The investigators found, however, that asthma risk factors such as smoking during pregnancy, passive smoke exposure after birth and a family history of asthma were more common among the late preterm infants.

When they analyzed for potential confounders in the late preterm birth group, the hazard ratio (HR) for maternal smoking during pregnancy contributing to development of asthma was 3.05 (95% confidence interval [CI] 1.86-5.01). The HR for passive smoke exposure after birth was 1.99 (95% CI 1.29-3.07) and the HR for family history of asthma was 3.26 (95% CI 2.22-4.78).

"After controlling for all pertinent covariates, we found that the association between late preterm births and the risk of asthma was significantly attenuated and lacked statistical significance," Voge and colleagues reported.

The key finding of the study, the investigators emphasized, was that maternal smoking during pregnancy is a modifiable environmental exposure that accounted for the association between late preterm births and the risk of asthma.

"This confounder must be included in any study that assesses the association of preterm birth and asthma," Voge and colleagues said.

This retrospective study collected information regarding smoking during pregnancy and passive smoke exposure after birth from medical records, so data on the timing and amount were imprecise. The investigators call for prospective studies in the future that can focus on the timing of smoke exposure in utero to better define the association with development of childhood asthma.

The birth cohort study was published in the March-April issue of Allergy and Asthma Proceedings. What Accounts for the Association Between Late Preterm Births and Risk of Asthma?

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