Placenta previa is a potentially life-threatening condition for both mother and infant. As such, antenatal diagnosis is essential to adequately prepare for childbirth.
Placenta previa has an incidence of 1/200 pregnancies.
Placenta previa is associated with a number of risk factors, including:
- previous placenta previa
- previous cesarean section
- increased maternal age
- increased parity
- large placentas
- maternal history of smoking
- assisted conception 6
- previous manual removal of placenta
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
The term "placenta previa" covers a spectrum of anomalies and results from the partial or total insertion of the placenta into the lower uterine segment.
Previa 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 0.5-2.0 cm from internal os).
- grade II: marginal previa: placental tissue reaches the margin of the internal cervical os, but does not cover it
- grade III: partial previa: placenta partially covers the internal cervical os
- grade IV: complete previa: placenta completely covers the internal cervical os
Sometimes grades I and II are termed a "minor" or "partial" placenta previa, and grades III and IV are termed a "major" placenta previa 5.
Due to placental trophotropism, the diagnosis of a placenta previa 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 centimeters of the cervical 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 in 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 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).
- 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.
- transvaginal ultrasound scan is more accurate to assess placenta previa, the transabdominal scan may overdiagnose 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
- 6. Royal College of Obstetricians and Gynaecologists (RCOG) (2011) Green Top Guideline No. 63: Antepartum Haemorrhage, 1st edition at https://www.rcog.org.uk/globalassets/documents/ guidelines/ gtg63_05122011aph.pdf
- 7. Jenny A. Cresswell, Carine Ronsmans, Clara Calvert et-al. Prevalence of placenta praevia by world region: a systematic review and meta‐analysis. (2013) Tropical Medicine & International Health. 18 (6): 712. doi:10.1111/tmi.12100 - Pubmed
Related Radiopaedia articles
- 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
- non-visualization of the fetal stomach
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- choroid plexus cysts
- enlarged cisterna magna
- shortened fetal long bones
- echogenic intracardiac focus (EIF)
- echogenic fetal bowel
- aberrant right subclavian 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
- fetal ductus venosus flow assessment
- umbilical venous flow assessment
- uterine artery flow notching
- nuchal translucency
- 11-13 weeks antenatal scan
- chorionic villus sampling (CVS) and amniocentesis
- first trimester and early pregnancy