The process of placental “migration” or relative upward
shift of the placenta due to differential growth of the lower
segment is continuous into the late third trimester.
15,18,27
Of
26 patients scanned at an average of 29 weeks’ gestational
age when the placenta lay between 20 mm away from the
internal os and 20 mm of overlap, only 3 (11.5%) required
CS for placenta previa at delivery. An average migration rate
of > 1 mm per week was highly predictive of a normal outcome. An overlap of > 20 mm after 26 weeks was predictive of the need for CS.
27
Predanic et al.
28
have subsequently
published similar results