Asoma is an unusual type of hemivertebra distinct from lateral hemivertebra in its underlying mechanisms, its rarity, its often isolated nature and the high risk of it causing medullary compression. We report a case of isolated partial agenesis of a vertebral body (asoma) diagnosed at 23 weeks' gestation by ultrasonography, in a fetus showing hyperkyphosis with vertebral canal disruption. The diagnosis was established using ultrasound and computed tomography, which allowed measurement of the angle of kyphosis and helped in the determination of the postnatal orthopedic prognosis. In our patient, prenatal diagnosis of asoma facilitated optimization of postnatal management and reduced the neonate's risk of developing paralysis. The main anatomical parameters in the determination of the orthopedic risk are the level of the anomaly, the angle of kyphosis and the degree of vertebral canal disruption.