Prenatal Ultrasound Diagnosis of Placenta Accreta: 2015

Prenatal Ultrasound Diagnosis of Placenta Accreta

Dr. Dolores Pretorius presents a tutorial on recognizing placenta accreta spectrum (PAS) on prenatal ultrasound. The condition can be missed—even by experienced imagers—so a technically adequate, systematic examination is essential.

What Placenta Accreta Spectrum Is

Placenta accreta occurs when placental tissue is abnormally attached to or invades the uterine wall and cannot be separated normally:

• Accreta: Placental villi are directly attached to the myometrium.
• Increta: Placental tissue extends into the myometrium.
• Percreta: Placental tissue grows through the uterine wall and may extend into nearby structures, often the bladder.

The condition is thought to relate to a defect in the decidua basalis and is commonly associated with prior uterine surgery. Because delivery can involve severe hemorrhage, treatment may require cesarean hysterectomy.

Risk Factors

Risk factors that should prompt a careful sonographic evaluation include:

• Prior uterine surgery, especially cesarean section or myomectomy
• Prior uterine instrumentation, such as dilation and curettage
• Placenta previa
• Advanced maternal age
• Grand multiparity
• Conception through in vitro fertilization

Although diagnosis is usually made in the second or third trimester, accreta may be suspected in the first trimester.

Normal Anatomy and Examination Setup

The normal placenta is generally homogeneous in appearance and more echogenic than the myometrium. The myometrium may range from nearly black to shades of gray and can contain small vessels.

A key examination requirement is to include the placenta–uterus interface and bladder in the image. The bladder provides an important landmark for assessing possible invasion at the lower uterine segment. The presenter recommends examining the interface in more than one view and changing the transducer angle when needed.

MRI can be used as an adjunct when ultrasound findings are ambiguous. Features discussed include placental bulging, myometrial thinning, placental lucencies, dark T2 placental bands, and loss of the normal tissue interface.

Ultrasound Findings to Look For

1. Low implantation in the first trimester

   • A gestational sac implanted unusually low in the uterus may raise concern for accreta.
   • Compare its position with the cervix and uterine fundus.

2. Placental lakes or lucencies

   • Multiple irregular spaces can give the placenta a “motheaten” appearance.
   • Their presence should prompt a review of the patient’s risk factors and closer assessment of the placental interface.

3. Myometrial thinning

   • Thinning may be subtle and may be the only early sign.
   • The presenter recommends a low threshold for follow-up when thinning is suspected.
   • Use a light transducer touch: pressure can compress the myometrium and mimic thinning.
   • Color Doppler may help distinguish a very dark but normal myometrium from an abnormal finding.

4. Irregular placental interface or interrupted serosa

   • Look for focal loss of the normal bright serosal line, especially at the bladder interface, but also elsewhere along the placenta.
   • Placental bulging toward or into the bladder can be concerning.
   • Assess in multiple views, as a single view may not reveal the abnormality.

5. Abnormal color Doppler flow

   • Suspicious findings include chaotic flow and vessels extending vertically into the myometrium or toward the bladder.
   • Compare the suspected area with other parts of the same placenta; abnormal and relatively normal regions can coexist in one patient.
   • When placental lakes are seen, the presenter recommends evaluating the interface along the entire placenta.

Common Pitfalls and Practical Checks

• An incomplete image can prevent exclusion of accreta. In particular, an image that does not show the bladder interface may be inadequate.
• Myometrial vessels can mimic placental lakes. Use additional views to determine whether spaces are within the placenta or represent vessels in the myometrium.
• Bladder curvature can cause refractive shadowing that resembles loss of the serosa. Change the transducer position or angle to determine whether the apparent break is real.
• One image is not enough. Move through the area and assess it in different planes, especially when findings are subtle or conflicting.
• Placenta previa alone is not accreta. A preserved myometrium and intact serosal interface support previa without visible accreta.

The tutorial ends with quiz examples that reinforce these points, including recognizing low implantation, identifying placental lucencies, detecting focal loss of the interface, distinguishing artifact from true disruption, and judging whether Doppler flow is suspicious.

Speakers and Sources Featured

• Dr. Dolores Pretorius — Presenter; Professor of Radiology at the University of California, San Diego, and Director of Fetal Imaging at the UCSD Maternal Fetal Care and Genetics Center.
• Allison Match — Credited by Dr. Pretorius as assisting with the presentation.