Maternal smoking during pregnancy and the risk for clubfoot in infants : North Carolina, 1999-2003

Statistical Brief North Carolina state center for health statistics State Center for Health Statistics www.schs.state.nc.us/SCHS/ March 2007 Maternal Smoking During Pregnancy and the Risk for Clubfoot in Infants: North Carolina, 1999-2003 by Kathryn C. Dickinson, Robert E. Meyer, and Jonathan B. Kotch* Introduction Talipes equinovarus, or clubfoot, is one of the most common major birth defects, with a preva¬ lence of approximately 1 per 1,000 live births.1'3 Clubfoot affects males about twice as often as females.4'5 Infants born with clubfoot have abnormal foot bones, ankle bones, foot muscles and ligaments, and their heel cords are often tight, preventing the development of a normal gait. These malformations require medical treatment and often surgery for correction. The causes of clubfoot are not well understood, although many have been proposed. A partial list of these causes include: intrauterine compression, vascular insufficiency, and neurologic abnormali¬ ties.6'8 Genetic and environmental factors are thought to play a role as well.91" Maternal smok¬ ing during pregnancy is one environmental risk factor that has been studied with varying results, with findings ranging from a positively-correlated dose-response effect to a protective effect.1112 This population-based case-control study aimed to further explore the link between maternal smok¬ ing during pregnancy and clubfoot in infants. It was hypothesized that maternal smoking during pregnancy would be associated with an increased risk of clubfoot being present in infants. Statistical Brief No. 31 North Carolina Department of Health and Human Services North Carolina Division of Public Health Public Health Methods This study used data from the North Carolina Birth Defects Monitoring Program (NCBDMP) matched to the Composite Linked Birth File. The Composite Linked Birth File contains all North Carolina resident birth certificates linked to Medicaid paid claims and health department service data, such as the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC) and maternity care coordination. Infants born in 1999-2003 and diagnosed with clubfoot were ascertained by the NCBDMP. Cases included live-born, singleton infants with a diagnosis of either isolated (non-syndromic) talipes equinovarus or clubfoot not otherwise specified (British Pediatric Association codes 754.500 and 754.730). Non-isolated cases, which were excluded from the analysis, were those where there was also a major congenital anomaly in another organ system, or another major muscu¬ loskeletal defect not related to clubfoot, such as a limb reduction defect. After excluding the non¬ isolated cases (about 35%) and those with missing maternal smoking data, a total of 443 cases were eligible for the study. •Kathryn Dickinson and Robert Meyer are with the North Carolina Birth Defects Monitoring Program. State Center for Health Statistics. Jonathan Kotch is with the Department of Maternal and Child Health. UNC School of Public Health.