In our daily practice, 2D TVS is essential in defining the relation between the lower placental edge and the IO in cases of low-lying placenta and placenta previa. This relation is fundamental in differentiating these types and for decision-making regarding the mode of delivery in such risky cases [14, 15]. In a previous case report, different measurements for EOD were reported by two sonographers [16]. However, the reproducibility of EOD measurement by 2D TVS and the inter-observer variability were not studied. Moreover, the conflicting results of the different studies about the cutoff EOD above which vaginal delivery can be attempted in these cases raises the suspicion of the inaccuracy of 2D TVS measurement of the EOD [10, 12, 13].
Theoretically, using 2D TVS, we localize the IO as the uppermost point of the cervical canal in the midsagittal view of the cervix. This would be the case if the cervical canal was tubular in shape and the IO has a pinhole appearance. However, in all cases, we found that the cervical canal and the internal os appeared as a slit, in the axial view of the cervix, surrounded by an oval hyperechogenic area representing the cervical mucosa (previously described by Simon and his colleagues as an “oval patch”) [16]. So, it is impossible to guarantee that the view in 2D examination of the cervix is strictly midsagittal, which may lead to errors in measurement of EOD; being nearer or farther from the placental edge (Fig. 9). Moreover, upon rotation of the vaginal probe to get the shortest EOD, both the IO and the lower placental edge must be visualized all through the movement, which becomes impossible upon reaching 90° lateral rotation on both sides. This is specifically important in cases of laterally located placentas. Therefore, another method was needed for more accurate spatial localization of the midpoint of the IO, and for simultaneous visualization of the IO and the lower placental edge during the rotation all around the IO to get the shortest EOD accurately.
The new method of EOD measurement by 3D TVS in the current study has achieved these goals. We could accurately localize the midpoint of the IO, and by positioning the reference point at this location, we could rotate the volume all around the IO while visualizing the lower placental edge to measure the actual shortest EOD whatever the placental location was. From a technical point of view, the most important steps were to place the reference point midway in the slit shaped internal os in plane C, and in the lowest level of the lower uterine segment in planes A and B. In plane B, the whole cervical canal was not visualized in all cases after manipulations, as this canal was curved in most cases and not always perpendicular to the lower uterine segment at the level of IO. However, this was not an essential prerequisite to complete the steps of measurement.
In the current study, the mean EOD measured by 3D TVS was significantly shorter than that measured using the 2D TVS, with dramatic increase in number of cases with EOD ≤ 10 mm and cases with EOD 11–20 mm measured by 3D TVS (Table 4). The most likely explanations of this difference are, firstly, the incorrect localization of the midpoint of the internal os (being farther from the placental edge) in 2D TVS and, secondly, the inability to simultaneously visualize the IO and the nearest point of the lower placental edge in a laterally located placenta.
As IO width ranged from 6 to 23 mm in this study, this can make a significant difference in measurement when there is marked shift from IO center. This was confirmed by the significant positive correlation between the IO width and the degree of difference between the EOD measured by both methods (Table 7). In seven cases, EOD measured by 3D TVS was longer than that measured by 2D TVS, mostly due to shift from the IO center towards the lower placental edge during 2D EOD measurement.
The difference in EOD measurement by 2D and 3D TVS was also related to the placental location. It was highly significant in anterolateral/posterolateral and lateral locations, being the highest with lateral group (Tables 5 and 6). This supports our hypothesis that the ability of 2D TVS to accurately measure the EOD decreases as the placenta is more lateral in location being almost impossible in directly lateral locations.
In their case report, Simon et al [16], described a different method of measuring EOD using 3D TVS, and the difference between 2D and 3D measurements was sufficient to shift from planned vaginal delivery to scheduled cesarean section for their case. They used multiplanar, omniView and surface rendered modes to accurately localize the center of the IO and to simultaneously visualize the whole lower placental edge and the IO.