In developed countries, smoking during pregnancy appears to be a major determinant of birth weight. In addition, it may increase the risk of congenital abnormalities. Maternal blood levels of folate are reduced in parturients who smoke. There is evidence that exposure to cigarette smoke compromises maternal folate status through altered dietary intake, and possibly through nondietary effects such as increased turnover of folate. The goal of this study was to learn whether and how maternal smoking influences plasma levels of 5-methyltetrahydrofolic acid (5-MTHFA) in umbilical cord blood from 58 pregnant African American women. Demographic features including smoking status were ascertained in structured interviews.
Venous cord plasma 5-MTHFA concentrations correlated with maternal plasma 5-MTHFA concentrations, although the venous cord levels were significantly higher. After controlling for maternal plasma 5-MTHFA concentrations, cigarettes smoked by the mothers at the first prenatal visit correlated negatively with venous cord 5-MTHFA concentrations. Both of these correlations were confirmed by multivariate analysis. Prepregnancy smoking was negatively associated with venous cord 5-MTHFA, and the mean 5-MTHFA concentration in venous cord plasma was significantly lower in the 32 mothers who smoked (15.1 ng/mL) than in the 26 nonsmokers (19.0 ng/mL). Prepregnancy smoking status did not correlate significantly with maternal plasma 5-MTHFA levels. Maternal smoking significantly influenced birth weight when controlling for gender and gestational age.
These findings suggest that maternal smoking may impair folate transport to the fetus, and may help to explain why folic acid fortification of foods seems to have a suboptimal effect in preventing neural tube defects in certain populations such as inner-city African American women.