BackgroundFetal growth is determined by its genetic potential and growth support. The aim of our prospective study was to find out does the growth support (placental factor and blood flow) or genetic potential play a primary role in fetal growth.MethodFrom 27 to 41 gestational weeks an umbilical artery (RI1), a middle cerebral artery resistance index (RI2) and umbilical‐cerebral (U/C) ratio were measured. Several US measurements in 3–4 week intervals of the same pregnancy were done. After delivery, fetuses were divided into the three groups (AGA, LGA, SGA) according to their birth‐weights. Pregnancies with gestational diabetes were excluded from the study.ResultsThere were not statistically significant differences between the average values of RI1 (age related) in the groups of AGA and LGA fetuses. However, the mean values of RI1 (according to gestational age) in the mentioned two groups of fetuses were significantly lower than in the group of SGA fetuses, as well as their mean values of U/C ratios.ConclusionPlacental factor is equally and efficiently functioning in the both groups of AGA and LGA fetuses. The genetic regulating factor of fetal growth can be adequately expressed in conditions of appropriate uteroplacental and umbilical blood supply. In other words, in identical or at least similar conditions of umbilical blood supply, fetal growth is exclusively influenced by its genetic potential.