A critical assessment of ultrasonic fetal cephalometry

The use of ultrasonic pulse echo techniques to measure the biparietal diameter (BPD) of fetal heads in utero and to monitor fetal growth rate has now become a routine procedure. Various claims as to the accuracy of ultrasonic cephalometry have been made. In an effort to reduce systematically the error in prediction of gestation age, an ultrasonic investigation has been made of aborted fetuses with a view to explaining the existence of a range of midline images. Our work suggests that whilst a midline is a necessary condition for BPD determination it is not, in general, a sufficient one. We have been able to demonstrate a variety of midlines corresponding to differing diameters of the fetal head at various levels of the fetal skull. Variations in acoustic impedance over the plane containing the falx cerebri are postulated to explain the appearance and position of the midlines. Beam divergence is adduced to explain our further observation that even when the sound strikes the fetal skull at angles other than normal incidence an image with a midline may nevertheless be found. A modification of the Campbell technique of determining biparietal diameters is proposed.