Asymmetric and parasitic conjoined twins are extremely rare anomalies of monochorionic monoamniotic twins, consisting of an incomplete twin attached to the fully developed body of the co-twin. Sonographic examination is essential for prenatal diagnosis as early detection of fetal anomalies with poor prognosis provides a chance to deliver the infant vaginally for the mother. We herein describe a case with a prenatal diagnosis of a parasitic twin using three-dimensional (3-D) ultrasound. The clear images obtained with 3-D ultrasound helped in counseling the parents. Making an early prenatal diagnosis of severe fetal anomalies with careful and sophisticated sonographic examinations should be warranted.