Objectives: To develop a sonographic dating formula based on fetal crown-rump length (CRL) in a Hong Kong Chinese population, quantify its systematic prediction error and compare its performance with established dating formulae.
Methods: This was a prospective observational study of women with a spontaneously conceived singleton pregnancy and a regular menstrual cycle in the preceding 12-month period. Ultrasound examinations were performed at gestational ages ranging from 6 to 15 + 6 weeks according to menstrual dates. The CRL of each fetus was measured three times, the mean of which was used to derive the best-fit fractional polynomial regression model for estimation of gestational age in relation to CRL. For each fetus, the menstrual age was compared to the gestational age calculated using three established dating formulae based on CRL measurement. The mean of the differences between estimated and menstrual age was calculated for each formula.
Results: Of the 394 subjects recruited one was excluded as the pregnancy resulted in intrauterine demise. The best-fit equation for the sonographic estimate of gestational age (GA, in days) from CRL (in mm) was GA = 26.643 + 7.822 x CRL(1/2)(R(2) = 0.96). The mean difference between menstrual age and the predicted gestational age was 0.22 days (95% CI, - 0.14 to 0.56), which was lower than that of the three established CRL dating formulae.
Conclusion: We have derived a formula suitable for the dating of naturally conceived pregnancies between 6 and 15 weeks of gestation that has no systematic prediction error (the 95% CI of mean difference between predicted and menstrual age included zero), comparing favorably with established CRL dating formulae.