Does age potentiate the smoking-related risk of fetal growth retardation?

Whether smoking is more harmful in certain pregnancies than others is a matter of controversy. In a large prospective study, interactions between smoking and other risk factors for small-for-gestational-age (SGA) were studied. Practically all Swedish live births between 1983 and 1985 were included, 280,809 births in all. In single births, significant interaction between maternal age on one hand and moderate (1-9 cigarettes/day) or heavy (ten cigarettes or more per day) smoking on the other, were obtained. The relative risk of SGA for heavy smokers versus non-smokers was 1.9 in the lowest age group (15-19 years) and 3.4 among women aged 40-44. Although smoking was twice as common among teenagers as compared to women aged 40-44, the attributable risk of smoking for SGA was higher among the older women. No other significant interactions with regard to fetal growth were obtained. In Sweden, there has been a shift towards delayed childbearing, and smoking during pregnancy is prevalent. These socio-demographic changes and the increased risk for SGA among older smokers must be considered when evaluating antenatal programs aimed at surveillance of fetal well-being.