Spectrum of abnormal placental villous adherence

Last revised by Dr Jeremy Jones on 20 Sep 2021

The spectrum of abnormal placental villous adherence describes the degree to which there is an invasion of chorionic villi into the myometrium because of a defect in the decidua basalis.

  • placenta accreta:
    • the commonest type of placental invasion (~75% of cases)
    • occurs in ~1 in 7,000 pregnancies
    • combination of previous C-section and an anterior placenta previa raises the probability of a placenta accreta
    • maternal mortality of up to 7% depending on location
  • placenta increta:
    • ~25% of cases
    • occurs in ~1 in 50,000 pregnancies
  • placenta percreta:
    • ~5% of cases

The incidence of all forms of abnormal placental villous adherence is increasing, which is felt to be due to the increased practice of cesarean sections

Placental villous adherence is classified on the basis of the depth of myometrial invasion:

  • placenta accreta:
    • mildest form 
    • villi are attached to the myometrium but do not invade the muscle
  • placenta increta:
    • intermediate form
    • villi partially invade the myometrium
  • placenta percreta:
    • severest form
    • villi penetrate through the entire myometrium or beyond serosa 

Definite imaging features can vary depending on the extent of invasion.

  • may show placenta previa
  • may show placental lacunae
    • variably-sized vascular structures in the placenta creating a "moth-eaten" or "Swiss cheese" appearance.
    • can be seen as parallel linear vascular channels extending from placental parenchyma into the myometrium: they tend to show turbulent flow (cf. placental venous lakes show laminar flow)
  • abnormal color Doppler
    • turbulent flow, i.e. disruption of the normal continuous color flow in the myometrium
    • increased vascularity is seen in the myometrium, and even in the urinary bladder in cases of placenta percreta
  • loss of retroplacental clear space
  • reduced myometrial thickness: anterior myometrial thickness <1 mm

MR images may show:

  • placenta previa
  • uterine bulging
  • heterogeneous signal intensity within the placenta
  • dark intraplacental bands on T2 weighted images
  • focal interruptions of myometrial wall
  • tenting of the urinary bladder
  • direct visualization of the invasion of pelvic structures by placental tissue

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Cases and figures

  • Figure 1: normal adherence
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  • Figure 2: placenta accreta
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  • Figure 3: placenta increta
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  • Figure 4: placenta percreta
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