We report a case of a twin pregnancy, wherein one twin presented with an abdominal cyst since 12 weeks' gestational age. Upon referral at 21 weeks' gestational age, three-dimensional ultrasound with Fly thru technology was used to aid in the identification of the etiology and nature of the mass. Once megacystis was confirmed, serial vesicocentesis and urine biochemistries were used to direct the management. This shows the potential of Fly thru technology in aiding the clinician in studying fetal congenital anomalies. This can help guide the diagnosis and provide earlier and timely management of such cases.