Z‐Score Reference Ranges for Umbilical Vein Diameter and Blood Flow Volume in Normal Fetuses

Objectives To establish Z‐scores for the diameter and blood flow volume of the umbilical vein (UV) in normal fetuses. Methods This was a prospective study involving 907 normal fetuses. We measured the diameter (Duv) of two different segments of the UV (FUV: the free loop of the UV; FIUV: the fetal intra‐abdominal UV). Next, we calculated the blood flow volume (Quv). Z‐scores were created for both Duv and Quv using gestational age, femur length, and biparietal diameter as independent variables. Results We successfully acquired 858 (94.6%) normal fetal measurements. Between 20 and 39 weeks, the Duv of the FUV and FIUV increased from 0.38 to 0.80 cm and from 0.33 to 0.70 cm, respectively. The Quv of the FUV and FIUV increased from 32.66 to 381.88 ml/min and from 31.50 to 360.15 ml/min, respectively. Linear or quadratic regression models were best fitted between the parameters of UV and the independent variables. Z‐scores were successfully determined for both the Duv and Quv. Conclusions The calculation of Z‐scores for the Duv and Quv is simple by applying standard statistical methods. These Z‐scores may be useful to evaluate placental circulation and provide a rationale for monitoring and evaluating the prognosis of fetuses.