Objective: To establish reference intervals for fetal umbilical vein (UV) diameter, UV time averaged maximum velocity (TAMXV) and UV flow (UVBF) at 11 + 0 to 13 plus; 6 weeks of gestation and to determine the intraobserver reliability of these measurements.
Methods: We considered, in a cross-sectional study, 162 fetuses from uncomplicated singleton pregnancies at 11 + 0 to 13 + 6 weeks of gestation. UV diameter and TAMXV were measured in its intraabdominal portion by real-time and Doppler ultrasonography and UVBF calculated. Relationships between these parameters and fetal crown rump length were tested by regression analysis. In 25 cases UVBF was measured twice by the same investigator and the intraobserver agreement of these measurements was calculated.
Results: A significant increase was evidenced in the gestational age period considered in UV diameter (r = 0.729, p < 0.0001), TAMXV (r = 0.691, p < 0.0001) and UVBF (r = 0.777, p < 0.0001). UVBF increased from 2.26 ml/min at 11 weeks to 15.62 ml/min at 13 + 6 weeks of gestation. The intraobserver ICCs were 0.96, 0.92 and 0.88, respectively, for UV diameter, TAMXV and UVBF.
Conclusions: In normal fetuses, UV diameter, TAMVX and flow increase between 11 + 0 and 13 + 6 weeks of gestation. In this gestational age interval, the calculation of UV blood flow and its components shows an intraobserver agreement reliable enough for clinical use.