Subamniotic hematoma is a placental pathological lesion resulting from the rupture of allantochorionic vessels. Some of them decrease in size during pregnancy. However, the relation between the reduction of the lesions during pregnancy and the structure of the placenta at delivery has not been previously reported. We report a case of subamniotic hematoma detected prenatally, which decreased in size during the third trimester, and presented with a cystic structure resembling a deflated balloon at delivery, and also review the literature of the placental surface cysts detected by prenatal ultrasound.