Introduction. The aortic isthmus is a specific feature of fetal circulation. It undergoes regression by becoming wider postpartum. Developmental abnormalities of an isthmus may lead to congenital defects. Aim. The aim of this work was a morphometric study of the aortic isthmus in human fetuses aged between 4 and 8 months of fetal life. Materials and methods. We investigated 223 human fetuses, including 108 males and 115 females, aged between 4 and 8 months of fetal life. The entire material was obtained from the Department of Histology and Embryology at the Collegium Medicum, Nicolaus Copernicus University in Bydgoszcz, Poland. All fetal specimens had been conserved in a 9% formaldehyde solution for a period of more than 3 months. Only spontaneously aborted fetuses with a normal morphology and a normal karyotype were used in this study. We investigated the diameter of the aortic isthmus in human fetuses at different stages of prenatal life. We also analyzed the ratio of that diameter with regard to the diameters of other segments of the aorta and ductus arteriosus. We considered how these measurements varied depending on sex. Results, Discussion and Conclusions. We found the growth of the aortic isthmus diameter to be linear in time. The measured diameters were similar in males and females. No significant differences with regard to sex were found between the ratios of that vessel’s diameter to the diameters of the ascending and descending aorta and the ductus arteriosus. The relative ratio of the aortic isthmus diameter to the diameters of the ascending and thoracic aorta decreased with time. On the contrary, the ratio of the aortic isthmus diameter to the ductus arteriosus diameter increased over time.