Maternal-fetal urea concentration difference in man: Metabolic significance

Abstract

TtIERE ~ A S ~,~EN considerable confusion within fetal physiology regarding the difference in concentrations of urea in maternal and fetal plasmas. In man, Plass and Matthew 1 reported equal concentrations of urea in maternal and fetal plasma. More recently, McGaughey and associates 2 reported similar findings of an equal concentration of urea in maternal and fetal blood. They suggested that the placenta was not required for excretion of urea of fetal origin. In contrast, Rubaltelli and Formentin, s studying normal full-term infants, reported consistently higher urea concentrations in cord blood than in maternal blood? The direction of a positive concentration difference for urea across the placenta is important, not only qualitatively in determining the pathway for excretion of urea produced by the fetus, but also quantitatively. Recent studies have provided information on the placental clearance rate of urea in sheep ~ and subhuman pri-