ObjectiveTo establish the Z-score reference ranges of fetal arterial ductus (DA) inner diameter of normal fetus at 20 to 40 weeks’ gestation.MethodsA cross-sectional study was conducted in 781 normal singleton pregnancies with 20-40 weeks of gestation who underwent fetal echocardiography in the Third Affiliated Hospital of Henan University of Chinese Medicine from November 2018 to September 2019. The DA diameter, biparietal diameter (BPD), and femoral diaphysis length (FL) were measured and used to estimate gestational age (GA). With BPD, FL, and GA as independent variables and DA as dependent variables, linear regression analyses were conducted on the mean and standard deviation of each parameter to construct the best Z-score models.ResultsThe linear regression equations reflected strong positive correlations between the fetal biological growth parameter BPD, FL, GA, and fetal DA inner diameter. The heteroscedasticity of standard deviations was eliminated by the weighted regression of absolute stagger, and the best Z-score models were constructed. The linear regression equations of GA (week), BPD (mm), FL (mm), and DA inner diameter (mm) were as follows: Y=−2.661+0.217×GA, Y=−2.505+0.084×BPD, Y=−1.734+0.100×FL; the linear regression equations of GA (week), BPD (mm), FL (mm), and the standard deviation of DA inner diameter (mm) were as follows:Y=−0.273+0.025×GA, Y=−0.440+0.013×BPD, Y=−0.180+0.012×FL. According to the models, Z-score normal range reference values of fetal DA inner diameter were calculated.ConclusionsThere are good linear relationships between fetal DA inner diameter and GA, BPD and FL. The Z-score reference ranges of fetal DA diameter during mid- and late-pregnancy are constructed, which are helpful to quantitatively evaluate the normal development of fetal DA and provide a reference for clinical diagnosis of fetal DA abnormality.