Accreta placentation

Myometrial thinning due to scar dehiscence

• This is not a strong ultrasound marker for PAS
• The myometrial thinning can be also seen when the placenta develops underneath a major scar defect (scar dehiscence). In scar dehiscence the myometrium is completely replaced by scar tissue

Myometrial thinning due to scar dehiscence is more pronounced in the third trimester when the lower uterine segment is further stretched. The myometrium can become so thin that the placenta can be seen through it at delivery. This phenomenon should be described as a ‘uterine window’ as it represents deficient myometrium rather than abnormal placentation.

In an invasive accreta the placental bladder border becomes irregular, whereas in scar dehiscence the placental bladder border is smooth and regular, as shown below.