Microscopically, the placental villi interdigitate directly with uterine myometrium, without an intervening decidual plate. This is placenta accreta, which will result in lack of normal separation at delivery, leading to marked hemorrhage necessitating emergency hysterectomy. The classification of placental invasion into myometrium is as follows:

Type	Invasion of chorionic villi has occurred...	The result is bleeding so severe that the obstetrician has to...
Accreta	Superficially into myometrium	Change socks
Increta	Deep into myometrium	Change pants
Percreta	Through the myometrium	Change underwear

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