For Physicians. By Physicians.™

FREE weekly ObG Insider NewsletterClick here

Does MRI Help or Hurt When Making a Diagnosis of Placenta Accreta?


  • Incidence of Placenta Accreta Spectrum Disorder is rising  
    • Estimated at 1/500-1/300 pregnancies  
  • Ultrasound is the standard radiologic modality, while MRI remains controversial  
  • Einerson et al. (AJOG 2018) sought to determine if MRI contributes to the sonographic diagnosis of Placenta Accreta Spectrum Disorder


  • Retrospective cohort study  
  • Participants 
    • Patients undergoing both ultrasound and MRI during 2nd and 3rd trimesters
    • Placenta Accreta Spectrum Disorder suspected (ultrasound or risk factors)  
  • Ultrasound risk factors 
    • Numerous echolucent placental lacunae | Loss of a normal retroplacental hypoechoic space | Loss of detectable myometrium | Bladder wall irregularity | Presence of abnormal subplacental vascularity 
  • Other clinical risk factors (if no suggestive US findings) included  
    • History of endometrial ablation or cavity-entering myomectomy | ≥3 cesarean deliveries in the setting of placenta previa | Suboptimal visualization of the placenta by US 
  • MRI decision left to MFM/surgical and radiology providers  
  • Diagnostic accuracy was verified by surgical and histopathologic diagnosis at the time of delivery 
  • Primary outcome  
    • Change in diagnosis from sonographic interpretation that could alter clinical management 
  • Secondary outcomes 
    • Correlation of radiologic diagnoses with surgical and histopathologic diagnosis


  • 78 patients were included   
  • Diagnosis that could alter clinical management occurred in 36% of cases 
  • MRI correctly  
    • Changed diagnosis in 19%  
    • Confirmed diagnosis in 44% 
  • MRI incorrectly  
    • Changed diagnosis in 17% 
    • Confirmed diagnosis in 21% 
  • MRI was not more likely to change a diagnosis in the 24 cases of posterior and lateral placental location compared to anterior location (33% vs 37%, P = .84) 
  • MRI resulted in overdiagnosis in 23% and in underdiagnosis in 14% of all cases 
  • In 14 severe Placenta Accreta Spectrum Disorder (percreta) cases, MRI altered only 2 diagnoses, both downgraded  
    • One was a correct downgrade to Placenta Accreta Spectrum Disorder (accreta and increta) and the other was an incorrect downgrade  
  • PPV for severe Placenta Accreta Spectrum Disorder 
    • MRI: PPV 61% (95% CI, 0.41–0.78) 
    • Ultrasound: PPV 73% (95% CI, 0.45–0.91) 
  • Proportion of accurate diagnoses with MRI did not improve over time despite increasing volume and increasing numbers of changed diagnoses


  • The addition of MRI to the assessment of Placenta Accreta Spectrum Disorder can often lead to an incorrect diagnosis  
  • The authors advise that MRI should not be used routinely as an adjunct to ultrasound in the diagnosis of Placenta Accreta Spectrum Disorder

Learn More – Primary Sources: 

Magnetic resonance imaging is often misleading when used as an adjunct to ultrasound in the management of placenta accreta spectrum disorders

Latest SMFM Guidelines: Third Trimester Bleeding Between 34w0d and 36w6d Gestation


SMFM provides guidance on the management of patients who present with bleeding in the late preterm period (34w0d to 36w6d).  The following are the key highlights and recommendations:

Placenta Previa

  • Stable and no other obstetric complications: Deliver between 36w0d to 37w6d (Grade1B)
  • Mild late preterm bleeding with 1 or more prior bleeding episodes that occurred <34 weeks of gestation: Consider delivery due to risk of recurrent bleeding
  • Mild bleeding 34 to 35 weeks with resolution by time of evaluation: Management is less clear
  • Do not perform routine cervical length screening to determine who will bleed in late preterm period as data in limited on appropriate management (Grade 2C)

Placenta Accreta

  • Definition: Abnormal trophoblast infiltration beyond the fibrinoid Nitabuch layer
    • Placenta increta: Placenta invades myometrium
    • Placenta percreta: Placenta invades beyond the myometrium
  • Incidence: <1% (in absence of placenta previa unless > 5 prior cesareans
  • Risk factors
    • Placenta previa and previous cesarean (most common)
    • Uterine surgery | Advanced maternal age | Smoking | Multiparity
  • Stable: Deliver between 34 to 37 weeks (Grade 1C)
    • If patient is stable, it is reasonable to briefly delay delivery to coordinate requisite multidisciplinary team
  • ACOG/SMFM recommendations (2019)
    • Delivery for suspected accreta, increta or percreta at 34w0d to 35w6d

Vasa Previa

  • Definition: Placental implantation that overlies or abuts the internal cervical os
  • Presentation: Painless bleeding
  • Incidence
    • Seen in 1 to 4% of second trimester ultrasound exams
    • 10 to 20% of previas diagnosed at 20 weeks gestation will remain a previa in the late 3rd trimester
  • Stable: Deliver between 34 to 37 weeks (Grade 1C)

Placental Abruption

  • Definition: Placental separation, either partial or complete, prior to delivery
  • Incidence: 0.5 to 1%
  • Classic presentation
    • Abdominal pain and bleeding
    • Nonreassuring fetal heart rate tracing (approximately 60%)
  • Risk factors: Hypertension | Smoking | PPROM | Cocaine abuse | Uterine myomas, and previous abruption
  • No clinical trials regarding ideal timing for delivery
    • Stable with high clinical index of suspicion: Delivery in late preterm or early term (expert opinion)
    • Diagnosis unclear, minimal bleeding, both mother and fetus stable: Delivery may be delayed with close surveillance and ongoing fetal testing
    • Active bleeding: Delivery as with any other active hemorrhage case


Delivery Considerations

  • If patient actively bleeding, delivery is indicated if the following are present
    • Significant vaginal bleeding
    • Abnormal laboratory results including such as acute anemia or coagulopathy
    • Abnormal fetal heart tracing
    • Maternal status unstable
  • If actively hemorrhaging, do not delay delivery for purpose of administering antenatal corticosteroids (Grade 1B)
  • Do not perform fetal lung maturity testing in late preterm period to guide management if there is an indication for delivery (Grade 1B)
  • Administer antenatal corticosteroids if (Grade 1A)
    • Delivery expected within 7 days
    • Gestational age is between 34w0d to 36w6d
    • Antenatal corticosteroids have not previously been administered
  • Cesarean section for placenta previa, vasa previa or accreta
    • For other clinical scenarios, vaginal delivery may be appropriate if
      • No contraindication for vaginal delivery
      • Fetal status is stable
    • Small amount of late preterm bleeding that has resolved by the time the patient presents may be treated expectantly if the following conditions met (no evidence-based recommendations currently available)
      • Both mother and fetus stable
      • Absence of active bleeding or contractions
      • Patient lives close to the hospital

Ultrasound Evaluation

  • Perform ultrasound exam to evaluate placental location prior to digital vaginal exam
    • Placental previa: Use transvaginal ultrasound
    • Vasa previa: Pulsed-wave Doppler may help identify a fetal arterial vessel (with FH rate) or fetal vessels with venous flow
  • Placenta accreta: Ultrasound can be used, but sensitivity (89 to 92%) and specificity (92 to 97%) less than that of placenta previa and vasa previa
  • Placental abruption: Use clinical suspicion/judgement to determine management as ultrasound can miss this diagnosis in 20 to 50% of the cases
  • MRI in women who are actively bleeding is not recommended

Laboratory Evaluation

  • Depends on clinical status and may include
    • CBC and platelets
    • Type and cross
    • Coag studies: PT/PPT/INR/fibrinogen
    • If transfusion likely: BUN, Cr and lytes
  • Wall clot test
    • Place blood in plain (red top) tube
    • Normal expectation is clot within 6 min
  • Rh negative patient
    • Assess maternal-fetal hemorrhage
      • Quantitative rosette test
      • Qualitative Kleihauer-Betke stain
      • Flow cytometry
    • Administer standard Rh immunoglobulin dose of 300 μg
      • Increase as needed based on quantitative testing

Initial Stabilization for Delivery

  • 2 large-bore intravenous lines
  • Obtain results from lab testing above, especially blood type
    • O-neg blood may need to be identified and prepared in the interim
  • Crossmatch for an initial 2 to 4 U of blood
  • Utilize hemorrhage protocol in units where available
  • Fetal heart monitoring is indicated

Evidence Grading System

  • 1A: Strong recommendation | High-quality evidence
  • 1B: Strong recommendation | Moderate-quality evidence
  • 2C: Weak recommendation | Low-quality evidence

Learn More – Primary Sources:

Society for Maternal-Fetal Medicine (SMFM) Consult Series #44: management of bleeding in the late preterm period

ACOG SMFM Committee Opinion 831: Medically Indicated Late-Preterm and Early-Term Deliveries

Placenta Accreta Spectrum Disorder: Definitions and Management

In normal circumstances, the trophoblast stops invading the uterus when Nitabuch’s layer is reached in the decidua. In cases of accreta, the trophoblast invades the myometrium due to a deficient or damaged Nitabuch’s layer. Placenta accreta occurs within a spectrum of disorders now referred to as ‘Placenta Accreta Spectrum’ (formerly ‘Morbidly Adherent Placenta’)

  • Placenta Accreta: trophoblasts attach to the myometrium without intervening decidua
  • Placenta Increta: trophoblasts invade through the myometrium
  • Placenta Percreta: trophoblasts move through the myometrium and invade beyond the serosa and into surrounding tissues

Clinical Actions:

  • It is advisable to refer women with clinical or ultrasound risk factors for placenta accreta to a center of excellence for evaluation, confirmation and delivery (Grade 1B – Strong recommendation – Moderate quality evidence)
    • Arrange for delivery at a level III or IV center with experience if possible
  • Optimal timing of delivery
    • Without bleeding or contractions can have a planned cesarean delivery at 34w0d – 35w6d (Grade 1A – Strong recommendation – High quality evidence)
    •  Consider delivery around 34w0d for women with the following (International Society for Abnormally – Invasive Placenta; Grade D recommendation)
      • History of previous preterm birth | Multiple episodes of small amounts of vaginal bleeding | A single episode of a significant amount of vaginal bleeding | PPROM
  • Corticosteroids
    • Administer corticosteroids to all women with suspected accreta if (SMFM recommendation Grade 1A)
      • Delivery expected within 7 days and meets gestational age criteria
      • Antenatal corticosteroids have not previously been administered
    • Do not delay delivery in the setting of active hemorrhage for the purpose of administering antenatal corticosteroids (SMFM recommendation Grade 1B)
  • Hospitalization vs outpatient care
    • Outpatient: In the absence of bleeding and any other symptoms or complications, there is limited evidence that hospitalization is of benefit
      • If outpatient management is elected, there must be a system in place for the patient to rapidly return to the hospital in case of bleeding, contractions or complications
    • Inpatient: women with preterm labor, PPROM or bleeding are “most likely to benefit” from hospitalization
  • Hgb levels (FIGO, RCOG and International Society for Abnormally – Invasive Placenta)
    •  If <110 g/L (11 g/dL) before 28 weeks or <105 g/L (10.5 g/dL) after 28 weeks
      • Work up for anemia and if indicated, begin iron supplementation (oral or intravenous) to optimize levels


  • In the setting of hemorrhage, the following blood product ratio range is recommended based on data from other surgical specialties (Grade 1A)
    • Packed RBC:FFP:Platelet = 1:1:1 to 1:2:4
  • High Risk management can be found in the ‘SMFM Guidelines’ and ‘Obstetric Care Consensus’ and International Society for Abnormally – Invasive Placenta ‘Evidence Based Guidelines’ (see ‘Learn More – Primary Sources’ below)
  • The International Society for Abnormally – Invasive Placenta does make the following recommendations (Grade D evidence)
    • Ureteral stents may be used, but insufficient evidence to recommend for routine use with primary benefit likely in the case of percreta
    • No evidence that routine vertical skin incision is superior to transverse and therefore base decision on
      • Placental location | Degree of possible invasion suspected |  Complication risk | Maternal body habitus | Gestational age | Preference of the surgeon
    • Avoid placental transection upon uterine incision even if this entails an upper segment or fundal incision
    • Ultrasound mapping (sterile) “of the exposed uterus should be used, where possible, to locate the placental edge and assist decision making regarding the uterine incision site”
  • Uterine preservation
    • Conservative management: Removal of placenta or uteroplacental tissue and no hysterectomy
      • Focal placental adherence
      • Followed by repair of uterine defect
      • Data limited
    • Expectant management: Placenta left in situ
      • More extensive adherence
      • Data limited
    • ACOG/SMFM obstetric care consensus (2018) states

Conservative management or expectant management should be considered only for carefully selected cases of Placenta Accreta Spectrum after detailed counseling about the risks, uncertain benefits, and efficacy and should be considered investigational (Grade 2C – Weak recommendation – Low quality evidence)


The incidence of placenta accreta has been increasing from 0.8/1000 in the 1980’s to 3/1000 deliveries. The risk increases with the increasing number of cesarean deliveries. This is especially true for women with placenta previa and prior cesarean sections. Mortality may be as high as 6 to 7%. Maternal complications are primarily the result of massive hemorrhage which can lead to DIC, multi-organ failure, hysterectomy, thromboembolism and death. Neonatal complications are the result of prematurity. The average gestational age at delivery of a pregnancy with Accreta is 34-36 weeks.

Key Points:

If previa is present

  • 3% risk of accreta with the 1st cesarean section
  • 11% risk of accreta at the 2nd cesarean section
  • 40% risk of accreta at the 3rd cesarean section
  • 61% risk of accreta at the 4th cesarean section
  • 67% risk of accreta at the 5th cesarean section

Risk factors

  • Prior cesarean section
  • Uterine curettage
  • Prior myomectomy
  • Placenta previa
  • Short inter-pregnancy interval
  • IVF
  • Multiparity
  • Prior pelvic radiation
  • Endometrial ablation
  • Maternal smoking

The mainstay of antenatal diagnosis is ultrasound

Note: Absence of ultrasound findings does not mean the patient does not have accreta | Clinical risk factors should be weighted equally compared to sonographic findings (Grade 1A)

  • First trimester findings include
    • Cesarean scar pregnancy (risk may approach 100% if pregnancy allowed to continue)
    • Gestational sac implanted in the lower uterine segment
    • Irregular vascular spaces in the placental bed
    • Loss of retroplacental-myometrial zone
  • Second and third trimester findings include
    • placenta previa (present in 80% of accreta)
    • loss of retroplacental-myometrial hypoechoic zone
    • multiple vascular lacunae
    • placental villi extension into the myometrium and beyond
    • interruption of the uterine serosal/ bladder interface
    • decreased retroplacental myometrial thickness (<1mm)

Role of MRI

  • MRI has sensitivities (75%-100%) and specificities (65%-100%) approaching that of ultrasound, but has not been shown to improve the diagnostic accuracy of accreta compared to ultrasound (Grade 1B)

Learn More – Primary Sources:

ACOG/SMFM Obstetric Care Consensus: Placenta Accreta Spectrum

Society for Maternal-Fetal Medicine (SMFM) Consult Series #44: management of bleeding in the late preterm period

SMFM Publications & Guidelines: Placenta Accreta

ACOG SMFM Committee Opinion 831: Medically Indicated Late-Preterm and Early-Preterm Deliveries

Abnormal Placentation: Placenta Previa, Vasa Previa, and Placenta Accreta

Evidence-based guidelines for the management of abnormally-invasive placenta: recommendations from the International Society for AIP

RCOG: Placenta Praevia and Placenta Accreta – Diagnosis and Management

FIGO consensus guidelines on placenta accreta spectrum disorders: Nonconservative surgical management

FIGO consensus guidelines on placenta accreta spectrum disorders: Conservative management

Evidence-based guidelines for the management of abnormally invasive placenta recommendations from the International Society for Abnormally Invasive Placenta

SOGC Guideline 383-Screening, Diagnosis, and Management of Placenta Accreta Spectrum Disorders

Special Report of the Society for Maternal- Fetal Medicine Placenta Accreta Spectrum Ultrasound Marker Task Force: Consensus on definition of markers and approach to the ultrasound examination in pregnancies at risk for placenta accreta spectrum