Placenta accreta is abnormal attachment of the placenta to the uterine wall. Pathologically there is a diminished or absent decidua basalis.



Link to Etiology




In the first trimester:

The usual location of a normal early gestation is in the fundus or very occasionally in the lower uterine segment. The sac is surrounded by thick myometrium on all sides.


  • Placenta accreta (and percreta) does occur in the first trimester. It is usually discovered during dilatation and curettage when massive bleeding occurs due to placental invasion of the myometrium by placenta (1-9).
  • Individuals who are at risk for placenta accreta at term are also at risk for placenta accreta in the first trimester, i.e. scarring of the uterus by surgical incision is a risk factor in the first trimester as well as later in pregnancy.
  • It may present as a low-lying sac that appears to be attached to the anterior wall of the uterus or uterine scar. The myometrium was thin in the area of the scar to which the sac was attached.

·         As early as 16 weeks irregular vascular sinuses appear, which have turbulent flow within them.

·         The bladder wall may appear interrupted or have small bulges of the placenta into the bladder space.

·         Absence of the normal echolucent space between the placenta and myometrium is not a reliable sign by itself, since this space may be absent in normal patients with an anterior placenta.

·         Color Doppler will show that some of the placental sinuses traverse the uterine wall.

·         Magnetic resonance imaging has not yet been shown to aid in the diagnosis, but in the future, with improvement of resolution and shortened sequences, it should be particularly useful in identifying the patients that have placenta percreta

In the second and third trimesters:

  • The area of interest is the retroplacental hypoechoic space.
    • Normal = 1cm (seen after 18 weeks) (10).
    • Placenta accreta - absence or thinning of this hypoechoic zone especially in a patient with a low lying placenta or placenta previa.
  • Ultrasound criteria for placenta accreta (11).

1.    Thinning (<1 mm) or absence of the hypoechoic myometrial zone in the anterior lower uterine segment between the placenta and the echodense boundary zone representing the uterine serosa and posterior bladder wall.

2.    Thinning, irregularity, or focal disruption of the linear, hyperechoic uterine serosa / bladder wall complex.

3.    The presence of focal mass-like elevations or extensions of tissue, with the same echogenicity as placenta, beyond the uterine serosa.

4.    The presence of lacunar vascular spaces in the placenta. Although the cause of placental lacunae is unknown, many authors have found them to be predictive of placenta accrete (12-15). Visualization of lacunae had the highest sensitivity (79%) in the 15-20-week range and a sensitivity of 93% in the 15-40-week gestational age time frame. Twickler et al (15) made the additional point that not only did these intraplacental sonolucencies predict all cases of accreta (with three false positives) but also that the lacunae did not need to be near the area of invasion. Finberg and Williams found that the likelihood of placenta accreta increased with the number of lacunae (14).

     The lacunae:

·         have a moth-eaten appearance to the placenta.

·         usually, but not always, have turbulent flow within them,

·         irregular, often more linear rather than rounded and smooth bordered.

·         do not have the highly echogenic border that standard venous sinuses have.

·         Tornado-shaped flow of venous, arterial or mixed blood is typical.

·         These sinuses have been seen as early as 9 weeks' gestation (15). Not all large sinuses or vessels are associated with placenta accreta.

·         Large sinuses in patients who did not have placenta accreta can be occasionally seen (these sinuses are smooth in contour and quite round).


Bladder border

·         The border between the bladder and myometrium is normally highly echogenic and smooth.

·         In the case of placenta accreta, interruptions or bulging can occur and  Finberg and Williams (14) found that this is a specific sign, but not a sensitive one, i.e. interruptions or bulging are not present in every patient with placenta accreta. However, this can be a problem in that patients who have had a Cesarean section often develop increased vascularity in the space between the myometrium and the bladder, probably because the bladder flap is retracted before the incision is made into the uterus and because this area is exposed to blood products. Therefore, it would be important to differentiate between bulging due just to enlarged or increased number of vessels and actual growth through the myometrium.

·         Bulging is somewhat non-specific for placenta percreta involving the bladder and does not always predict these cases. In the three cases in our series in which this sign was present, two had a placenta percreta. Kirkinen et al. noted a bulging of the placenta into the bladder in two of their cases of placenta accreta but there was no growth into the bladder (16). Thus, it appears that bulging of the bladder wall may indicate accreta but does not diagnose percreta. Care must be taken to examine the bladder wall with the ultrasound transducer at 90° so that it is clearly seen.


Myometrial thickness

·         Twickler et al. measured the thickness of the lower uterine segment in women who had had a previous Cesarean section and had a low-lying anterior placenta or placenta previa by measuring between the bladder wall and the retroplacental vessels, as seen by color Doppler. All patients later proven to have placenta accreta had a myometrium of less than 1 mm (15), which was as predictive of accreta as lacunae. .


Loss of the clear space

·         The usual dark line between the myometrium and the placenta is thought to represent the decidua basalis. Since the decidua basalis is absent in placenta accreta, it has been suggested that the absence of this line suggests placenta accreta. This line may be absent in many normal patients with anterior placentas (17).



·         Lerner et al. found that turbulent blood flow extending from the placenta into surrounding tissues was very sensitive and correctly identified all patients with accreta. This finding was not present in any of the patients without accrete (18).

·         Levine et al., in a blinded study, found that power Doppler did not improve the diagnosis of placenta accrete (19).

·         Chou et al. used Doppler in the third trimester and thought that it was reliable. However, on close examination gray-scale images appeared to be as effective and color Doppler could not distinguish accreta from increta (20).







Normal retroplacental hypoechoic zone















Loss of the retroplacental hypoechoic zone
























Loss of the retroplacental hypoechoic zone which most likely corresponds to the decidua basalis, myometrium, and dilated venous channels (7,8)









Progressive thinning of the retroplacental hypoechoic zone on serial examinations

Normal hyperechogenic boundary line




Loss of the hyperechogenic boundary line between the uterine serosa and wall of the bladder


Multiple placental lakes

Multiple placental lakes that may represent dilated vessels extending from the placenta through the myometrium—the so-called "Swiss cheese" appearance of the placenta (9)

Thinning of the uterine serosa-bladder wall complex (percreta)

Focal mass-like elevation of tissue with the same echogenicity as the placenta beyond the uterine serosa (percreta) (1)

Elevation of tissue beyond the uterine serosa (percreta)



Color and Power Doppler Imaging

Detection rates of placenta accreta with color and power doppler imaging, especially anterior placenta accreta, have been high, because it can detect with a high level of confidence abnormal uteroplacental hypervascularity caused by the angiogenesis of placental invasion.

Dilated vascular channels with diffuse lacunar flow

A diffuse lacunar flow pattern from dilated vascular channels scattered throughout the whole placenta and the surrounding myometrial or cervical tissues. High velocity pulsatile venous-type flow can be found in the sonolucent vascular spaces. A finding of this nature has been associated with a higher level of maternal morbidity and mortality.


Irregular vascular lakes with focal lacunar flow

A focal lacunar flow pattern showing irregular sonolucent vascular lakes with turbulent lacunar flow distributed regionally or focally within the intraparenchymal placental area.

Hypervascularity linking placenta to bladder

Interface hypervascularity with abnormal blood vessels linking the placenta to the bladder




Dilated vascular channels

Markedly dilated peripheral subplacental vascular channels with pulsatile venous-type flow over the uterine cervix.


Poor vascularity at sites of loss of hypoechoic zone.

Absence of subplacental vascular signals in the areas lacking a peripheral hypoechoic zone.



3D Ultrasound










  • Massive vaginal bleeding (intra- and postpartum).
  • Uterine wall rupture.
  • Invasion of adjacent structures.
  • Uterine infection.
  • Uterine inversion due to attempted manual removal of the placenta (22-24).



  • 25% infant mortality rate (23,24).
  • 10% maternal mortality (23,24).






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