Placental transport capacity may increase as gestation advances due to an increase in the rate of extraction of substances from uterine or umbilical blood (Barcroft, 1946; Faber & Thornburg, 1983; Meschia, 1983). In fact, extraction of oxygen per unit of uterine blood increases from mid- to late gestation in sheep and cattle (Meschia, 1983; Reynoldset al.1986). Placental uptake is calculated based on the Fick principle: