Definition: A very rare benign tumor that may arise in many places, of which the most common is the oral cavity. The tumor is also called "granular cell myoblastomaâ€.
Prevalence: Extremely rare. This tumor occurs in females exclusively.
Etiology: It has been postulated that the etiology could be due to excessive production of estrogens by the fetal ovaries under hCG stimulation[1].
Ultrasound features:  A large solid mass protruding from the fetal mouth. Polyhydramnios is evident due to inability of the fetus to swallow[2]. Vascular connections between the tumor and oral floor may be demonstrated using Doppler technique.
Reference
[1] Cussen, L.J., MacMahon, R.A. Congenital granular cell myoblastoma. J Pediatr Surg 1975; 10:249-53
[2] Schwartz, Z., Katz, A., Zosmer, A., Dreasen, E., Lancet, M., Sagiv, M. Polyhydramnion caused by giant alveolar granular cell myoblastoma. Acta Obstet Gynecol Scan 1986; 65:519-521.