To explore the relevance of the rheological properties of maternal blood in pregnancy to fetal growth a study of whole blood viscosity (WBV) was conducted in the early third trimester of 138 pregnancies. A significant negative correlation was found between WBV at low shear rate and birthweight centile. As the rheological parameters were found to be negatively correlated with the placenta coefficient, an independent role for maternal WBV seems likely. When WBV, placental weight and degree of infarction accounted for significant contributions in a logistic regression model, diastolic blood pressure data did not assist in the correct prediction of occurrence of a low birthweight centile (less than 10th). A simplified model is proposed, to explain the mechanisms by which some clinical variables may express their influence on fetal growth. In conclusion, it is suggested that WBV might be considered one of the factors which determine the efficacy of placental perfusion on the maternal side. However, as this variation in efficacy of placental perfusion is only weakly reflected in variations in birthweight, the influence of WBV on fetal growth cannot be very important.