Associations between placental cadmium and zinc and age and parity in pregnant women who smoke.

We have previously shown that the ratio of placental zinc to placental cadmium (Zn/Cd ratio) is positively related to infant birth weight in pregnant smokers. Clinical studies have reported that older pregnant smokers are at higher risk for impaired fetal growth than younger pregnant smokers. This study examines the relationships among placental cadmium, placental zinc, placental Zn/Cd ratio, age, and parity in 98 smokers and 151 nonsmokers. Atomic absorption spectroscopy was used to analyze cadmium and zinc. Thiocyanate was used as an index of smoking status. The data were analyzed using univariate correlation and repeated-measures analysis of variance. The results showed that increased parity is related to increased levels of placental cadmium in smokers, and decreased placental zinc in smokers and nonsmokers. Age is inversely related to the Zn/Cd ratio in both smokers and nonsmokers; moreover, the oldest nonsmokers have a higher ratio than the youngest smokers. These results are consistent with a depletion of body zinc stores with increasing parity and the long half-life of cadmium in the body. The data explain in part the clinical finding that smoking during pregnancy is more harmful in older women.