Abstract
Ultrasound scanning in the first trimester of pregnancy occasionally revealed a thin linear echo in the gesta-tional sac. A total of 360 examinations, which had shown a live fetus with a crown-rump length of 7–82 mm, equivalent to a menstrual age of 6–14 weeks, were reviewed. The linear echo was seen in 51 of the examinations, at an ultrasound age of 6–12 weeks. Most frequently it was seen in the 9th week, where it was recorded in 28 of 61 (46%) of the examinations. The origin of the linear echo is believed to be the very thin amniotic membrane, which from the 8th to the 12th week is lined by fluid on both sides. Other intra-uterine membranes which all give stronger echo reflections are listed.