Placenta previa

Placenta previapraevia 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 to both mother and infant, which may result in exsanguination. As such, antenatal diagnosis is essential to adequately prepare for childbirth.  

Pathology

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

Classification

PreviaPraevia is divided into 4 grades depending on the relationship and distance to the internal cervical os:

• grade I: low lying placenta: placenta lies in 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 previapraevia: placental tissue reaches the margin of the internal cervical os, but does not cover it
• grade III: partial previapraevia: placenta partially covers the internal cervical os
• grade IV: complete previapraevia: placenta completely covers the internal cervical os

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

Risk factors

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

• previous placenta previapraevia
• previous caesareanCaesarean section
• increased maternal age
• increased parity
• large placentas:
  • multiple gestations
  • erythroblastosis
• maternal history of smoking

Associations

• abnormal placental villous adherence (~5%): e.g placenta accreta

Radiographic features

Ultrasound

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

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

MRI

MRI is the gold standard to imaging 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.

Treatment and prognosis

A low-lying placenta is relatively common at the mid-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 previapraevia, a caesarianCaesarian section is required for delivery to avoid the risk of fetal and maternal haemorrhage.

Differential diagnosis

• full bladder
• focal myometrial contraction

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

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