Abstract: Massive subchorionic thrombosis is a rare condition, defined as a large thrombus confined to the subchorionic space. It is associated with poor perinatal prognosis. However, prenatal diagnosis by ultrasonography is often difficult. We report a case of massive subchorionic thrombosis developing dermatomyositis after the delivery, followed by magnetic resonance imaging. Moreover, we review other 4 cases assessed with magnetic resonance imaging. Magnetic resonance imaging is very useful for confirmation of diagnosis and follow-up in combination with ultrasonography.