Abstract. The importance of longitudinal changes in maternal Whole Blood Viscosity (WBV) for fetal well-being was investigated. Consecutive WBV data were available, obtained from 44 pregnancies before 36 0/7 weeks. An increase in WBV was found to be associated with an unfavorable fetal outcome. This result might be considered as clinical evidence supporting the hypothesis that maternal WBV contains information on the efficacy of placental perfusion.