Abnormal Placental Location
Roya Sohaey, MD
DIFFERENTIAL DIAGNOSIS
Common
Placenta Previa
Marginal Sinus Previa
Placenta Accreta Spectrum
Less Common
Succenturiate Lobe
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
Rule out lower uterine segment (LUS) placentation in 2nd and 3rd trimesters
Transabdominal routine images
Midsagittal view
Parasagittal views
Perform transvaginal ultrasound (TVUS) if LUS not seen with routine views
Bleeding is not a contraindication
Use careful technique
Watch screen while inserting probe
Perform translabial/transperineal ultrasound if TVUS not possible
Collapsed vagina is acoustic window
Elevate maternal hips to minimize bowel artifact
Place probe on perineum (labia minora)
Is fetus or fluid in direct contact with cervix?
Placenta may block direct contact
Floating fetus in 3rd trimester
Transverse presentation
Does the uterus look asymmetrically thick?
Placenta & myometrium vs. area with only myometrium
Succenturiate lobes often missed
Placenta location assigned before uterus completely evaluated
Have a high index of suspicion for accreta
Suspect accreta if previa & prior cesarean section
↑ Risk with ↑ number of cesarean sections
Use color and pulse Doppler
Placenta accreta
Vessels may extend beyond myometrium
Succenturiate lobe
Vessels connect placentae
Rule out vasa previa
Pulse Doppler can help differentiate maternal from fetal vessels
Fetal vs. maternal heart rate
Helpful Clues for Common Diagnoses
Placenta Previa
Subtypes of PP are based on the distance between placenta margin and cervix internal os (IO)
Complete PP completely covers IO
Partial PP partially covers IO
Marginal PP within 2 cm of IO
Second trimester PP often resolves
Most PP seen < 20 wks resolve by 34 wks
5% PP incidence at 15-16 wks
0.5% PP incidence at term
Placental “migration” (trophotropism): Areas of placenta atrophy as others grow
LUS “stretches” later in pregnancy
Associated with placental abruption
Patient presents with bleeding
Placental edge lifted by hematoma
Mass-like hematoma seen by cervix
Associated with preterm labor
More common if also bleeding
Assess cervical length
Assess for IO distention/funneling
Cervical canal may be distended with blood
5% of PP will have associated accreta
Marginal Sinus Previa
Marginal sinus PP is a subtype of marginal PP
Evaluate marginal placental vein distance to IO
Placental vessels < 2 cm from IO
Veins are maternal, not fetal
Do not confuse with vasa previa
Placenta Accreta Spectrum
PA: Pathologic nomenclature based on depth of placental invasion
Accreta means myometrial attachment without muscle invasion
Increta means myometrial invasion
Percreta means invasion through uterus
Imaging does not differentiate between subtypes well
PP & invasion of cesarean section scar
↓ Subplacental hypoechoic myometrial zone (< 2 mm)
Vessels extending through myometrium
Distended placental lacunae
Bizarre large sonolucencies
“Tornado-shaped”
Often near PA site
MR findings
Loss of normal low signal myometrium with T2WI
Avoid gadolinium
Helpful Clues for Less Common Diagnoses
Succenturiate Lobe
SL: Accessory placental lobe or lobes
Often smaller than main lobe
Identify placental cord insertion site
Most often on main lobe
May be velamentous (between lobes)
Rule out vasa previa
Low SL
Crossing vessels cover IO
At risk for fetal hemorrhage
SL is often missed if the entire uterus not imaged
5% of all deliveries have SL
Most often asymptomatic and discovered at delivery
May present with bleeding if SL previa
May present as retained product of conception
Other Essential Information
High risk patients for abnormal placental implantation
Prior placenta previa
Prior cesarean section
Prior suction curettage
Prior uterine surgery
Advanced maternal age
Multiparity
Smoking
Cocaine use
Symptoms at presentation
Incidental finding
Painless bleeding
Fetal distress
Preterm labor
Multiple diagnoses often seen together
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
