The factor by which the villous surface area is enlarged owing to the presence of microvilli has been evaluated with quantitative analyses in human placental tissues from mid-gestation to term. It has shown that, between 25 and 36 weeks of gestation, the peripheral villous surface area is enlarged by a constant factor of approximately 9.47 +/- 0.28 (mean +/- s.d.). Then, from 36 weeks to term, it has shown a significant decrease in the microvillous surface enlargement factor (9.44 to 7.67; P less than 0.01). Consequently, the actual surface area of exchange between mother and fetus was shown to be significantly decreased during that same period (93.91 to 67.02 m2; P less than 0.01). On a functional basis, these findings support the theory that, during that last four weeks of pregnancy, the increasing physiological needs of the fetus are probably met by profound functional changes in the permeability and transfer functions of the cells that constitute the placental barrier.