Placenta increta, invasion of the myometrium by normal placental (chorionic) villi, can be life-threatening to a mother at delivery. In two reported cases of pathologically proven placenta increta, the abnormal areas were retrospectively identified on ultrasonographic images. In both cases, there was consistent focal obliteration of the hypoechoic retroplacental zone by either tissue of echogenicity similar to that of the normal placenta or tissue of slightly decreased echogenicity. This was found to represent extension of the villi through the decidua basalis into the myometrium. The spectrum of non-malignant invasive placentas and the specific ultrasonographic findings that should allow prospective diagnosis are discussed.