Maternal smoking and birth defects: validity of birth certificate data for effect estimation.

ABSTRACT

OBJECTIVES: The authors sought to assess the validity of birth certificate data for estimating the association between maternal smoking and birth defects. The US standard birth certificate includes check boxes for maternal smoking and for 21 congenital anomalies. The sensitivity and specificity of birth certificate data have been studied, but previous studies have not addressed the validity of these data for estimating the association between birth defects and maternal smoking or other risk factors. METHODS: US public-use natality data (1997-1998) were used to calculate the prevalence ratio (adjusted for maternal age, race/ethnicity, and education) for the association between maternal smoking and 13 defects/defect categories. All analyses were restricted to 45 states, New York City, and the District of Columbia because they collect both maternal smoking and birth defect data. RESULTS: Maternal smoking was associated with an increased prevalence of hydrocephaly (adjusted prevalence ratio [PR] = 1.24; 95% confidence interval [CI] = 1.08, 1.43), microcephaly (PR 1.47; 95% CI 1.15, 1.88), omphalocele/gastroschisis (PR 1.37; 95% CI 1.22, 1.53), cleft lip/palate (PR 1.35; 95% CI 1.25, 1.45), clubfoot (PR 1.62; 95% CI 1.49, 1.75), and polydactyly/syndactyly/adactyly (PR 1.33; 95% CI 1.23, 1.43 ). Previous studies have indicated an association between maternal smoking and gastroschisis, oral clefts, and clubfoot with effect estimates of similar magnitude to this study. CONCLUSIONS: These findings suggest that birth certificate data may be useful for exploratory or corroborative studies estimating the association between birth defects and some risk factors recorded on birth certificates.