CONCEPTUAL DATING BY ULTRASONIC MEASUREMENT OF THE FETAL BIPARIETAL DIAMETER IN EARLY PREGNANCY

The accuracy and precision of gestational dating, based on single measurements of the fetal biparietal diameter (BPD), was estimated in a consecutive series of 970 apparently normal, singleton pregnancies. The BPD sizes used varied between 11 and 60 mm, corresponding to 9–22 postconceptional weeks. The ‘conceptual ages’, used for reference, were estimated by means of ultrasonic measurements of the fetal crown—rump lengths (CRL). The association between the estimated conceptual age (dependent variable) and BPD size (independent variable) was found to be well represented (R2 = 0.972) by a second order polynomial: conceptual age = 44.7 + 1.069 × BPD+ 0.01382 × BPD × BPD. To check the validity of the proposed equation, we used 23 fetuses conceived by artificial insemination with donor semen as controls. There was good agreement between the virtual conceptual ages of the control cases and their corresponding BPD-estimated conceptual ages (0.5 days mean difference, 3.6 days SD) when the suggested regression equ...