Ultrasonography of the fetal neck in the first and early second trimesters. Part 1. Normal appearance

The soft tissues of the neck of 196 normal fetuses were examined with ultrasonography during the first and early second trimesters, and the criteria for normality are described here. The dorsal pseudomembrane is the most important feature of this area in a normal fetus. This feature appears as a single echogenic line lying parallel to the occiput and the upper cervical spine but separated from them by an anechoic area, which should be less than 2.8 mm deep before 18 weeks gestational age. The membrane is best observed when the fetus is in the "neck-up" position with the neck flexed. Although the superior-inferior extent of the membrane is variable, the occurrence of lateral extension, septations or hydrops or a bulbous appearance indicates abnormality. The membrane should move freely with the motion of the neck, which allows it to be differentiated from the normal amnion, amniotic bands or synechiae.