Ultrasonography of the normal fetal brain.

Since the early 1960s it has been possible to identify the fetal head using A-mode ultrasound. The introduction of high-resolution, real-time, gray-scale ultrasound in the mid-to-late 1970s first enabled the routine display of subtle internal anatomy. Using presently available equipment, the fetus can be imaged from 7 weeks after the last menstrual period until term and the internal anatomy of the fetal head can be detected after 11-12 weeks. The ultrasonic anatomy of the normal fetal head throughout the second and third trimesters is discussed, as are the normal changes taking place with growth.