There isn't much to google anyway, lol! From what I can gather it is often due to the placenta growing too far under the endometrium, spreading out laterally & pushing up a wall of tissue around it ("circumvallate" = encircled by a wall).
"A ‘shelf at the margin of the placenta’ is the terminology used to describe the sonographic appearance of tissue contiguous with the edge of the placenta, protruding into the uterine cavity. The tissue is iso-echogenic with the placenta and is not attached to any fetal parts. This shelf is considered to represent circumvallate placenta."
It also said that it tends to fix itself by the third trimester anyway!
A study from 2006 -
In 17 of 152 (11.2%) early second-trimester scans a placental shelf was detected. In three of these 17 cases the shelf persisted to the 20–22-week scan. In the two cases that presented for the third-trimester scan the shelf was no longer present. In all 17 cases the perinatal outcome was good.
Conclusion - In our study group early second-trimester placental shelves rarely persisted to mid-gestation and never to the third trimester. There were no placenta-related perinatal problems. Early second-trimester placental shelf appears to be a common, benign and transient sonographic finding. Copyright © 2006 ISUOG. Published by John Wiley & Sons, Ltd.
