Ultrasonic Fetal Cephalometry

Until ultrasonic technics became available, accurate measurement of the fetal head size in utero was not obtainable. Savage (1) and Mengert and Korkmas (2) were not satisfied with the adequacy of roentgenographic cephalometry, and Crichton (3), evaluating the accuracy and value of cephalopelvimetry, stressed the difficulty in arriving at precise roentgen biparietal diameter measurements. In 1961 Donald and Brown (4) published an article on the employment of ultrasound for measuring the size of the fetal head. Following their report several other authors, including Taylor et al. (5) and Anderson and Niswonger (6), used ultrasound for the determination of the biparietal diameter in utero and recorded results within 3 mm of the actual postnatal caliper measurements. The proved accuracy of the ultrasonic technic of cephalometry has served as a stimulus for other workers to adopt the procedure and utilize it as a supplement to current diagnostic x-ray studies. Experimental work has attested to the safety of ultrasonography (7), and extensive clinical use has failed to indicate any injurious effects or discomfort to the fetus or the mother. It is important that the transmitting beam be perpendicular to the interface to achieve a vertical reflection of the echo to the transducer. Should the beam strike a surface obliquely it will be deflected and the transducer will not receive a strong echo. Technic In performing ultrasonic fetal cephalometry, the transducer is placed on the maternal abdomen over the general area of the fetal head and moved around until a pair of strong and equal echoes are displayed on the screen. This indicates that the transmitting beam is perpendicular to two parallel interfaces, and, as the skull is so shaped that only the parietal areas are relatively parallel, it follows that they are responsible for reflecting the beam. The distance between the reflections represents the biparietal diameter, and the distinguishing feature of this diameter is the pair of strong echoes (Fig. 1). Some confusion might result if the sound path is in a true sagittal plane of the skull, but the magnitude of the anteroposterior dimension would serve to differentiate it from the biparietal diameter. The technic of ultrasonic cephalometry is quite simple to master, and with a short period of instruction and supervision an x-ray technician can become proficient in performing the study. Clinical Application So that clinical studies could be approached with confidence, a preliminary correlation of ultrasonic cephalometry with caliper measurement of the biparietal diameter was carried out, and our results confirmed the accuracy of ultrasonic fetal cephalometry claimed by others (4, 6). The diameters were measured ultrasonically in 75 cases, either before or during active labor; of these 17 were breech presentations, 2 were transverse, and 56 were vertex. Several of the patients suffered from diabetes and hypertension.