How is placenta praevia defined and classified?

Placenta praevia is a placenta implanted in the lower uterine segment, generally defined after 28 weeks (most low placentas 'migrate' up by 30-32 weeks as the lower segment forms).

• Major praevia - placenta covers the internal os
• Minor / low-lying - placental edge within 2 cm of the os but not covering it
• >2 cm from the os - not praevia

Clinical relevance. The major/minor classification determines the mode and timing of delivery and the accreta risk.

More Detail

Definition and classification of placenta praevia

Placenta praevia is implantation in the lower segment, and the degree determines management.

Definition and migration

• Placenta in the lower uterine segment; generally defined after 28 weeks.
• Most low placentas migrate up around 30-32 weeks as the lower segment forms and lengthens - so a low placenta at the mid-trimester scan is rechecked later.

Classification

• Major praevia - the placenta covers the internal os.
• Minor / low-lying placenta - the placental edge is within 2 cm of the internal os but does not cover it.
• More than 2 cm from the os - not placenta praevia.

Significance

The classification determines mode and timing of delivery (major praevia and bleeding minor praevia need caesarean), and warns of morbidly adherent placenta (accreta), particularly with a previous caesarean.