EC is a benign, developmental odontogenic cyst, which creates a pathological cavity with fluid, semifluid, or gaseous content. EC accompanies an erupting primary tooth and forms shortly before the tooth’s appearance in the oral cavity. The prevalence of EC has not been well studied. An extensive literature review showed a low prevalence of EC [6, 7]. In some surveys, an increased incidence of EC has been observed in Caucasians [2, 7, 8]. Most reports of ECs are in the first decade of life, and only a few studies have shown this type of lesion in newborns [8, 9]. To the best of our knowledge, there are have been no prenatal reports of EC. We observed one case of EC in an Asian fetus during prenatal screening, which was confirmed after delivery. Ultrasound may provide some information for the diagnosis of EC, including the extent and contents of the mass, which may affect the mode of delivery.
The origin of an EC is still controversial. Aguiló et al.  studied 36 cases and found that trauma, caries, infection, and less space for eruption were possible factors that may cause EC in older children. Some authors have attributed the origin of EC to degenerative changes in reduced enamel epithelium after the end of amelogenesis or have suggested that this cyst develops from the remnants of the dental lamina that coats the erupting teeth [2, 6]. On the basis of prenatal discovery of EC, we speculate that the latter histological interpretation was likely in our case.
Standard ultrasound screening views or targeted views can help to detect the mobility, extent of involvement, and tissue characteristics of this lesion. Mandibular central incisors and permanent first molars are the most common sites for EC . We found that an EC during the fetal period was detected as a lesion that comprised a solid mass and cystic fluid with low mobility. The homogeneous hyperechogenic mass of the lesion was finally determined to be a tooth. In the oblique coronal plane view, the signal was absent behind the tooth, which strongly absorbed ultrasonic waves, and acoustic shadowing of the tooth caused interruption of the echo on the palate . Therefore the artifact of EC on the palate can easily be incorrectly diagnosed as orofacial clefts. 3D imaging allows visualization of structures in planes that are difficult to obtain with conventional 2D sonography. This visualization is achieved by collecting a multiplanar reconstruction view displaying a series of 2D slices in three perpendicular orthogonal planes. However, poor image acquisition and visualization using 3D imaging occur when fetal movement or position does not allow adequate imaging of the specific area. Additionally, 3D ultrasound imaging is subject to the same types of artifacts seen in 2D images . Therefore, screening views should include multiple sections from different angles to avoid the artifacts.
Treatment options for EC include no treatment and follow-up or surgical extraction. EC may be associated with natal or neonatal teeth, such as in our case. When the cyst interferes with feeding, natal teeth have high mobility and/or are poorly developed, and they need to be removed.
In conclusion, ultrasound can reveal the distinctive features of fetal EC. The treatment required for EC is mainly determined by the symptomatology of the newborn. Prenatal ultrasonography can be used to identify the position and the size of the mass, which are considered to be useful for localized and qualitative diagnosis before birth.