Ultrasound findings using SMI
Ultrasound examinations in the present study were performed by two authors (NF and JH). The placenta and umbilical cord were evaluated using B-mode to detect gross abnormalities, such as velamentous cord insertion, hyper-coiled cord, and entanglement. The placenta was then evaluated precisely using SMI from the decidual layer to the chorionic plate. When some ultrasound findings were found, the exhibited findings were mapped to the respective location on the placenta based on the location and form of the placenta and the spatial relationship with the umbilical cord insertion site.
In the normal findings of the placenta using SMI, the villous trees are identified on the fetal side—from the umbilical cord insertion to the stem villi—connecting to the terminal villi into the maternal side. Tertiary stem vessels in the villous tree are homogenous and sharply diminished. Additionally, maternal blood flow from the spiral artery is observed as ’scatter’ in the intervillous space (Figure 1, Appendix S1) . In contrast, abnormal findings were defined as follows.
Infarction
Since both the villous blood vessels and intervillous space disappear, both the background ”scatter” and “villous tree” are not detected, thus, resulting in an echoic space (Figure 2a, Appendix S2).