Chinese Fetal Biometry: Establishment of a Formula for Calculating Gestational Age based on Crown–Rump Length Measurements

Abstract Background: To develop an ultrasonographic dating formula for predicting gestational age (GA) based on fetal crown–rump length (CRL) in a Chinese population, evaluate its systematic prediction error and compare it with existing formulae.Methods: This was a prospective cross-sectional study of spontaneously conceived singleton pregnancies among women with a regular menstrual cycle in the preceding year. Ultrasound examinations were performed at 11–14 weeks according to the date of the last menstrual cycle. The CRL was measured three times for each fetus, and the mean was used to derive the best-fit fractional polynomial regression model for estimation of GA in relation to CRL. For each fetus, the GA was compared with the GA calculated using six established dating formulae based on CRL measurements. The means of the differences between estimated and menstrual age were calculated for each formula. All the women were followed up routinely until the birth of the fetus. Results: Of the 4710 subjects recruited, the mean and standard deviation values of CRL changed linearly with GA. The corresponding regression equation and its correlation coefficient (R2) was GA = 59.361513 + 0.461425 ´ CRL (R2 = 0.8028). The mean difference between estimated and menstrual age was 0.22 days (95% confidence interval 0.05–0.21), lower than that of the six existing CRL dating formulae.Conclusions: We have derived a CRL-based dating formula suitable for naturally conceived pregnancies for GA between 11+0 and 13+6 weeks. The formula has no systematic prediction error, comparing favorably with the existing published dating formulae.