Obstetrics

Placenta Praevia

Placental tissue lying over the internal cervical os shears as the lower uterine segment forms, opening maternal vessels and causing painless recurrent bleeding from a segment that then contracts poorly.

Definition

Placenta praevia is placental tissue lying over the internal cervical os. A low-lying placenta has its leading edge less than 20 mm from the os. Both are ultrasound diagnoses, measured at the mid-pregnancy anomaly scan and re-measured later in pregnancy. The older grade I to IV system, and the split into major and minor praevia, is not used in current UK guidance.

First principles

The diagnosis is a measurement, not a grade

Placenta praevia means the placenta lies over the internal cervical os. Low-lying means the placental edge is less than 20 mm from the os. An edge 20 mm or more away is not low-lying. Current UK sources describe placental position by that distance alone, and assign no grade.

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