For a placental edge reaching or overlapping the internal os,
Mustafa et al.
21
found in a longitudinal study an incidence of
42% between 11 and 14 weeks, 3.9% between 20 and 24
weeks, and 1.9% at term. With overlap of 23 mm between
11 and 14 weeks, they estimated that the probability of placenta previa at term was 8%. Similarly, Hill et al.
22
reported
an incidence of 6.2% for a placenta extending over the
internal os between 9 and 13 weeks. In their series of 1252
patients, 20 (1.6%) had overlap of the placental edge of
16 mm or more, and only 4 of these had placenta previa
persisting to term (0.3%). Two additional studies that have
examined various distances of overlap between 9 and
16 weeks
23,24
agreed that persistence of placenta previa is
extremely unlikely if the degree of placental overlap is no
more than 10 mm. Two studies examined cut-off values at
18 to 23 weeks’ gestation.
25,26
These found a similar incidence of the placenta reaching or overlapping the internal
os of up to 2%, and overall less than 20% of these persisted
as placenta previa. The likelihood of persistent placenta
previa was effectively zero when the placental edge reached
but did not overlap the os (0 mm) and increased significantly
beyond 15 mm overlap such that a distance of > 25 mm
overlap had a likelihood of placenta previa at delivery of
between 40% and 100%.