Maternal Tobacco Use As a Risk Factor for Small for Gestational Age (SGA) Is a Third-Trimester Effect

BACKGROUND:   While small for gestational age (SGA) is a well-known consequence of maternal smoking, the magnitude of risk for SGA by week of gestational age has not been elucidated.  To assess that risk, we examined more than three million records in the 1990 to 2009 life birth certificate file for four central Appalachian states. 

METHODS:   Live births (N = 3,032,928) with birth weight, gestational age (22-44 weeks), and maternal tobacco use history were  categorized as SGA or not, based on 10^th percentile gender-specific weights-for-age (Oken et al., 2003).   SGA prevalence was analyzed for tobacco users and for tobacco non-users yielding relative risks and odds ratios. Gestational week-specific rates, rate differences, and multivariate logistic adjusted odds ratios were also analyzed.  

RESULTS:   SGA prevalences among tobacco users (19.5%) and among tobacco non-users (9.1%) yielded for SGA significant prevalence rate ratio and odds ratios of 2.1 to 2.4.  The pattern for SGA by gestational week was similar for comparative rates, rate differences, and adjusted odds rates.  The rate for tobacco non-users was steady near 9%.  The rate for tobacco users was steady until week 33 when it rose monotonically through week 37 to about 20% at week 38 and remained high.  Tobacco use was not seen to be a risk factor for SGA for second-trimester births.  The effect of tobacco use grew during the period of premature birth and became fully evident with a two-fold risk for full term infants. 

CONCLUSIONS: We newly report the temporal pattern of tobacco-related SGA by week of gestational age. Tobacco-related SGA was only seen in the third trimester – increasing during weeks 33 through 37 with a doubling during weeks 38-44.  This pattern is informative for issues of mechanism and demonstrates the benefit of maintaining tobacco cessation programs through the end of pregnancy