The Interpretation of Ultrasonic Echograms in Abdominal Pregnancy

Ultrasonic echoscopy provides a picture which resembles a black and white outline drawing. There is no “colour” information, some organs being shown in white and others in black or grey. The reason for this is that sound is reflected from interfaces between tissues of differing acoustic impedance and not from the substance of a homogenous tissue itself. For example, there is no significant difference between the impedance offered to ultrasound by normal placental tissue and that by myometrium so that the utero-placental interface does not return echoes which can be displayed on the screen of the echoscope. This is in contrast to the acoustic impedance of liquor amnii which differs from that of placental tissue allowing the interface between amniotic fluid and placenta to be outlined. Interpretation of echograms therefore must be on a basis of geometrical outline rather than on the definite identification of a landmark by its echo character. This principle is illustrated in echograms from a recent case of abdominal pregnancy from the Women’s Hospital.