Clinical manifestation of small-for-gestational-age risk pregnancy from smoking is gestational age dependent.

To determine the gestational age period at which small-for-gestational-age (SGA) risk from pregnancy smoking manifests, we conducted a retrospective cohort study of 266,782 live births in Kansas City, Missouri, from 1990 to 2004. Information was obtained from birth records. Newborns were stratified by gestation (< 32, 32 to 36, and > or = 37 weeks) and maternal pregnancy smoking. The outcome of interest was SGA. Covariates included factors associated with fetal growth restriction. SGA rates varied (8.4% versus 15.7% versus 9.9%), and relative risk from smoking increased (1.01 versus 1.46 versus 2.22) with gestational age periods. On multivariable logistic regression, smoking increased the odds ratio (OR) of SGA only among infants > or = 32 weeks; OR (95% confidence interval [CI], 1.26 [0.94, 1.68], 1.78 [1.59, 1.99], and 2.62 [2.52, 2.72]), for < 32, 32 to 36, and > or = 37 weeks, respectively. In conclusion, the clinical manifestation of SGA risk from smoking is gestational age dependent, only becoming apparent after 32 weeks of gestation.