Selenium and normal pregnancy The growing fetus requires selenium. Selenium is transported across the placenta by passive diffusion down a concentration gradient. Decidual selenium concentrations are higher than endometrial, whereas umbilical venous concentrations of selenium are lower than maternal In countries such as Poland and Yugoslavia in which soil selenium content and dietary intake are low, maternal selenium concen trations and GPx activity fall during preg- nancy, being the lowest at delivery com pared with nonpregnant controls. In contrast, in areas of very high soil selenium content (eg, South Dakota and Southern Spain), it would appear that there is no gestational trend in serum sele- nium concentrations.