Abstract The morphometric oxygen diffusive conductance ( D p) of the placenta provides a measure of the efficiency of oxygen transfer between the mother and the developing fetus. Any change in the D pmay point towards possible adaptation in the light of altered oxygen transfer. Placentae from normal ( n=40) and small for gestational age SGA ( n=24) pregnancies were analysed using stereological techniques. Each placenta was uniform randomly sampled and tissue samples processed to wax infiltration and embedding using conventional histological preparatory methods. A combination of stereological techniques and physiological constants were used to estimate the partial conductances across the five major tissue compartments involved in oxygen transfer. There was a significant reduction in both fetal birthweight and placental weight in the SGA group when compared with controls. A decrease in both chorionic ( S cv) and fetal capillary ( S fc) surface area was also observed in SGA placentae when compared with controls ( P>0.001). Villous membrane harmonic thickness ( T vm) was reduced in the SGA placentae (2.33 μm) when compared with controls (2.67 μm P=0.019). This resulted in a reduction in the minimum D pin SGA placentae when compared with controls ( P=0.023). Adjusting for fetal weight resulted in no difference in the specific diffusive conductance. Changes in T vmin SGA placentae combined with changes in basic surface areas were insufficient to maintain overall D pvalues comparable with control placentae.