The need for CS at term is predicated upon the os to placen-
tal edge distance and clinical features (e.g., presence of
unstable lie and/or bleeding). Five studies have examined
the likelihood of CS for placenta previa on the basis of dis
-
tance to the placental edge on the last ultrasound prior to
delivery.
6,27–31
The last scan was performed at a mean of 35
to 36 weeks’ gestational age, and a distance of > 20 mm
away from the os was associated with a high likelihood of
vaginal delivery (range 63–100%). It has been suggested
that this cut-off distance of > 20 mm away from the os
should be defined as a low-lying placenta, rather than a placenta previa, in order to avoid the bias of physicians performing elective section based on the report of a placenta
previa.
30
These cases can be managed in the high
expectation of a vaginal delivery.