Placenta praevia

Dr Daniel J Bell and Radswiki et al.

Placenta praevia is a term given to an abnormally low position of the placenta such that it lies close to, or covers the internal cervical os. 

It is a potentially life-threatening condition for both mother and infant, which may result in exsanguination. As such, antenatal diagnosis is essential to adequately prepare for childbirth.  

Placenta praevia is associated with a number of risk factors, including: 

  • previous placenta praevia
  • previous Caesarean section
  • increased maternal age
  • increased parity
  • large placentas:
  • maternal history of smoking

The term "placenta praevia" covers a spectrum of anomalies and results from partial or total insertion of the placenta into the lower uterine segment.

Praevia is divided into four grades depending on the relationship and distance to the internal cervical os:

  • grade I: low-lying placenta: placenta lies in the lower uterine segment but its lower edge does not abut the internal cervical os (i.e lower edge 0.5-5.0 cm from internal os).
  • grade II: marginal praevia: placental tissue reaches the margin of the internal cervical os, but does not cover it
  • grade III: partial praevia: placenta partially covers the internal cervical os
  • grade IV: complete praevia: placenta completely covers the internal cervical os

Sometimes grades I and II are termed a "minor" or "partial" placenta praevia, and grades III and IV are termed a "major" placenta praevia 5.

Due to placental trophotropism, the diagnosis of a placenta praevia is not usually made before 20 weeks.

During the 'routine' 18 to 21-week morphology scan, the distance between the lower edge of the placenta and the internal os should be measured. If it lies within a few centimetres of the os, then a repeat ultrasound at ~32 weeks should be performed to ensure that the edge has migrated further away. 

MRI is the gold standard imaging modality for the placenta and its relationship to the cervix, although in most instances it is not required. Sagittal images best demonstrate the relationship of the placenta to the internal cervical os.

A low-lying placenta is relatively common on the second trimester morphology scan. As the fetus grows and the uterus expands, the lower uterine segment thins and grows disproportionately, such that in most cases the placenta is no longer low-lying by a follow-up study (usually performed at 32-34 weeks).  

In the case of a complete placenta praevia, a caesarian section is required for delivery to avoid the risk of fetal and maternal haemorrhage.

These can make the placenta appear closer to the internal cervical os than it actually is (particularly on a 2nd trimester scan). Postvoid images should always be obtained if praevia is suspected.

Occasionally, a subchorionic haematoma that extends over the cervix can mimic placenta praevia, especially if the haemorrhage is still echogenic. Follow-up imaging would be useful to distinguish the two entities.

  • transvaginal ultrasound scan is more accurate to assess placenta praevia, transabdominal scan usually overdiagnoses it in up to one quarter of cases

  • when spotted in the second trimester, a third-trimester ultrasound scan (~32-34 weeks) should be performed to reassess the placenta position 

Ultrasound - obstetric
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Article information

rID: 12477
System: Obstetrics
Synonyms or Alternate Spellings:
  • Placenta previa
  • Placenta praevia spectrum
  • Placenta previa spectrum
  • Placenta previa general
  • Placenta previas

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Cases and figures

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    Figure 1: type I - low lying placenta
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    Case 1: type IV : complete
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    Figure 2: type II - marginal previa
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     Case 2: with vasa previa
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    Figure 3: type III - partial previa
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    Case 3: type IV - complete
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    Figure 4: type IV - complete previa
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