Abnormal fetal gastrointestinal system morphology Fetal oral mass 2024-03-08T06:51:53Z A swelling or enlargement localized to the oral cavity of a fetus. The word mass is usually used at an early stage of the diagnostic workup before the precise nature of the swelling has been determined. This finding is generally first observed by prenatal sonography or magnetic resonance imaging. Fetal oral masses are usually observed in the second and third trimesters.