Placenta praevia

Classification

• Classically: complete, partial and marginal placenta praevia, plus a low-lying placenta.
• The simplification used today is praevia when the internal os is covered, and low-lying when the edge is within 20 mm.
• The diagnosis is made transvaginally, which measures the distance to the internal os most accurately.

Clinical features

Risk factors and presentation

• Typical is sudden, painless, bright red bleeding, often waking her at night.
• The uterus is soft and not tender, and the CTG is usually normal at first.

Diagnosis

Measurement

• What is assessed is the wall (anterior, posterior, fundal, lateral) and the distance from the placental edge to the internal os.
• The most accurate is a transvaginal measurement with an empty bladder.
• A full bladder or a contraction of the lower segment can mimic praevia.
• The placental site belongs in the second screening scan of routine antenatal care.

"migration"

• A low-lying placenta in the second trimester resolves in the great majority of cases by the third.
• The reason is growth of the lower uterine segment, not true movement of the placenta.
• So a check is made in the third trimester, around 32 weeks.
• The further the placenta overlaps the os initially, the less likely it is to resolve.

What else is checked

• After a previous caesarean the relation of the placenta to the scar is assessed, because that is where the accreta risk lies.
• With a low-lying placenta you also look for vasa praevia with colour Doppler.

Typical imaging findings

• Ultrasound: The placenta lies on the posterior wall and extends across the internal os, which it completely covers. No free distance between placenta and cervix can be measured; the myometrium beneath the placenta is preserved.

Keep learning in the app

Further reading (selection)

1. StatPearls: Placenta Previa (NCBI Bookshelf)
2. DocCheck Flexikon, Placenta praevia

Cross-references