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.
Treatment and prognosis
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.
- full bladder
- focal myometrial contraction
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
- 1. Merz E, Bahlmann F. Ultrasound in obstetrics and gynecology. Thieme Medical Publishers. (2005) ISBN:1588901475. Read it at Google Books - Find it at Amazon
- 2. Dashe JS, Mcintire DD, Ramus RM et-al. Persistence of placenta previa according to gestational age at ultrasound detection. Obstet Gynecol. 2002;99 (5 Pt 1): 692-7. - Pubmed citation
- 3. Carol Benson MD. Ultrasonography in obstetrics and gynocology, a practical approach, Thieme. (2007) ISBN:1588906124. Read it at Google Books - Find it at Amazon
- 4. Elsayes KM, Trout AT, Friedkin AM et-al. Imaging of the placenta: a multimodality pictorial review. Radiographics. 29 (5): 1371-91. doi:10.1148/rg.295085242 - Pubmed citation
- 5. Impey L. Obstetrics & gynaecology. Wiley-Blackwell. (2004) ISBN:1405107219. Read it at Google Books - Find it at Amazon
- placental anatomy
- placental developmental abnormalities
- placenta previa
- spectrum of abnormal placental villous adherence
- abnormalities of cord insertion
- abruptio placentae
- placental pathology
- vascular pathologies of placenta
- placental infections
- placental masses
- molar pregnancy
- twin placenta
Ultrasound - obstetric
- ultrasound (introduction)
- obstetric ultrasound
first trimester and early pregnancy
- gestational sac
- yolk sac
- Beta-hCG levels
- ectopic pregnancy
- multiple gestations
- subchorionic hematoma
- failed early pregnancy
- fetal biometry
- fetal morphology assessment
- fetal echocardiography views
- nonvisualisation of the fetal stomach
- nuchal fold thickness
- absent nasal bone
- choroid plexus cysts
- enlarged cisterna magna
- shortened fetal long bones
- echogenic intracardiac focus (EIF)
- echogenic fetal bowel
- aberrant right sublavian artery
- fetal pyelectasis / fetal renal pelvic dilatation
- single umbilical artery
- sandal gap toes
- Doppler ultrasound
- umbilical artery Doppler assessment
- fetal middle cerebral arterial Doppler assessment
- ductus venosus flow assessment
- umbilical venous flow assessment
- nuchal translucency
- 11-13 weeks antenatal scan
- chorionic villus sampling (CVS) and amniocentesis
- first trimester and early pregnancy