Normal sonographic appearance of the fetal neck late in the first trimester: The pseudomembrane

A linear specular reflection simulating a membrane was visualized along the back of the neck on sonograms of ten normal fetuses ranging in age from 10 to 14 menstrual weeks. Follow-up ultrasound studies of eight of the fetuses and post-partum clinical examinations of all subjects were normal, revealing no evidence of neck abnormalities. Although this pseudomembrane is similar in appearance to published examples of early cystic hygroma, it is a normal finding that can be distinguished from early cystic hygroma on the basis of its less bulbous appearance and the absence of underlying septations. This structure corresponds in distribution to the skin along the back of the fetus and likely represents a specular reflection from the skin surface.