Placenta previa is an abnormally low-lying placenta covering the internal cervical os. As a common cause of antepartum hemorrhage, placenta previa is a potentially life-threatening condition for both mother and infant. As such, antenatal diagnosis is essential to prepare for childbirth adequately.  

Epidemiology

Placenta previa has an incidence of 1 in 200 pregnancies ^ref.

Risk factors

Placenta previa is associated with several risk factors, including ⁸: 

• previous placenta previa

• previous cesarean section

• increased maternal age

• increased parity

• large placentas

  • multiple gestations

  • erythroblastosis

• maternal history of smoking

• assisted conception ⁶

• previous manual removal of placenta

Associations

• placenta accreta spectrum disorders (~5%): e.g. placenta accreta ^ref

Clinical presentation

Placenta previa usually presents with painless vaginal bleeding in the second half of pregnancy (>20 weeks gestation), most commonly between 34-38 weeks gestation. Other associated clinical features include:

• high fetal presenting part

• maternal/fetal compromise secondary to exsanguination

Pathology

Previously, placenta previa was classified into four grades ⁵ but due to difficulty in separating grades (i.e. marginal coverage vs partial coverage) the following definitions to describe the relationship of the placental edge with the internal cervical os ⁸:

• placenta previa: placenta covering the internal os

• low-lying placenta: placenta <2 cm from the internal os but not covering it

• normal: placenta >2 cm from the internal cervical os

Radiographic features

Ultrasound

Due to placental trophotropism, the diagnosis of a placenta previa is not made before 16 weeks ⁸.

MRI

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.

Treatment and prognosis

A low-lying placenta is relatively common in the second-trimester morphology scan, as the fetus grows and the uterus expands, the lower uterine segment thins and grows at a faster rate ⁸, such that in most cases the placenta is no longer low-lying by a follow-up transabdominal ultrasound. In asymptomatic women, this is recommended at 32 weeks, and if the placenta is persistently low-lying (including placenta previa), a further follow-up transvaginal ultrasound is recommended at 36 weeks ⁸.

In the case of a complete placenta previa, a cesarian section is required for delivery to avoid the risk of fetal and maternal hemorrhage.

History and etymology

Previa is of Latin origin. It is a combination of 'prae' (meaning before) and 'via' (meaning way).

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 second trimester scan). Postvoid images should always be obtained if previa is suspected.

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

Practical points 

• transvaginal ultrasound scan is more accurate to assess placenta previa, the transabdominal scan may overdiagnose it in up to one-quarter of cases

• when diagnosed in the second trimester, a third-trimester ultrasound scan at 32 weeks should be performed to reassess the placenta position ⁸