Hemoconcentration in smoking mothers is associated with impaired fetal growth

Objective. To examine the effect of maternal smoking on the relationship between maternal hemoglobin levels and pregnancy outcome. Design. A prospective study of healthy parous women from early pregnancy and of their infants. Setting. Three Scandinavian university hospitals covering all deliveries from well defined geographical areas. Subjects. Smoking (669) and non‐smoking (368) mothers, para 1 and 2 and with >37 weeks of gestational length. Main outcome measures. Birth weight and placental weight. Ponderal Index and Placental Index as measures of possible discordant fetal and placental growth. Results. In non‐smoking mothers the hemoglobin levels in the three trimesters had no relation to birth weight. In smoking mothers a significantly lower birth weight was seen with a high hemoglobin level in the third trimester, but hemoglobin levels in early or mid‐pregnancy had no association to birth weight. Smoking mothers also had a significantly greater fall in hemoglobin concentration from first to second and third trimester as compared to non‐smokers although ferritin levels were similar in smokers and non‐smokers, implying similar iron stores. The ratio of placental weight to the weight of the newborn was significantly higher in smokers, but no association was found to different hemoglobin levels. Conclusions. Fetal growth impairment associated with maternal smoking is even more pronounced in smoking mothers with high hemoglobin levels in late pregnancy. Smoking mothers were also found to have disproportional fetal/placental growth with relatively high placental weights. In non‐smoking mothers hemoglobin levels had no relation to birthweight.