The mechanism of apparent placental movement is not completely understood. The term migration is clearly a misnomer, however, because invasion of chorionic villi into the decidua on either side of the cervical os persists.
The apparent movement of the low-lying placenta relative to the internal os probably results from inability to precisely define this relationship in a three-dimensional manner using two-dimensional sonography in early pregnancy. This difficulty is coupled with differential growth of lower and upper myometrial segments as pregnancy progresses. Thus, those placentas that “migrate” most likely never had actual circumferential villus invasion that reached the internal cervical os in the first place.
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