INTRODUCTION: The fetus receives oxygen and nutrition from the placenta through the umbilical vein. There is a large variation in placental size and placental size relative to fetal size (placental-fetal ratio). We studied whether placental size or placental-fetal ratio is related to blood flow in the umbilical vein. MATERIAL AND METHODS: We performed a prospective study at Akershus University Hospital in Norway and a total of 73 women were examined in gestational weeks 27 and 37. Fifty-six women were examined at both time points. B-mode ultrasound and Doppler ultrasonography were used to measure the umbilical vein volume blood flow per minute. Magnetic resonance imaging was used to measure placental and fetal volume. Pearson's and Spearman's correlation coefficients were applied for estimations of associations. RESULTS: In gestational week 27, the umbilical vein volume blood flow was positively correlated with fetal size (Pearson's correlation coefficient r = 0.372, p = 0.003, and Spearman's correlation coefficient r = 0.384, p = 0.002), but not with placental size (Pearson's r = 0.130, p = 0.317, and Spearman's r = - 0.53, p = 0.687) or placental-fetal ratio (Pearson's r = - 0.61, p = 0.641, and Spearman's r = - 0.218, p = 0.092). In gestational week 37, we found positive correlations of the umbilical vein volume blood flow with fetal size (Pearson's r = 0.428, p = <
0.002, and Spearman's r = 0.391, p <
0.002), placental size (Pearson's r = 0.400, p = 0.002, and Spearman's r = 0.643, p <
0.002), and with placental-fetal ratio (Pearson's r = 0.224, p = 0.066, and Spearman's r = 0.266, p = 0.028). CONCLUSIONS: A large placenta and a high placental-fetal ratio are associated with increased umbilical vein blood flow.