The need for CS at term is predicated upon the os to placental 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 distance 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.