The placenta establishes a connection between the mother and the fetus for nutrient and gas exchange. Normally, the placenta detaches cleanly from the uterine wall after childbirth. Placenta Accreta Spectrum (PAS) is a serious complication where the placenta abnormally adheres to or invades the muscular wall of the uterus, preventing normal separation. This deep attachment carries a significant risk of massive hemorrhage and other complications for the mother at delivery. Prenatal detection of this condition is a high priority, and the primary method for this surveillance is specialized ultrasound imaging.
Understanding Placenta Accreta Spectrum
The term Placenta Accreta Spectrum describes a range of conditions classified by the depth to which the placental tissue invades the uterine wall. The least invasive form is Placenta Accreta, where the villi attach directly to the superficial muscular layer of the uterus, called the myometrium, without penetrating it further.
The next level of severity is Placenta Increta, which occurs when the placental villi not only attach to but also invade into the thickness of the myometrium. This deeper penetration makes separation significantly more difficult and increases the likelihood of severe bleeding. The most severe form of the condition is Placenta Percreta, where the placental tissue penetrates entirely through the uterine wall.
In cases of Placenta Percreta, the placental tissue can extend beyond the uterus and invade adjacent organs, such as the urinary bladder or bowel. The increasing incidence of PAS is closely linked to the rising rate of cesarean deliveries, as prior uterine scarring is a significant risk factor. Precise classification of the invasion depth is necessary for planning a safe delivery and comprehensive maternal care.
Ultrasound as the Primary Screening Tool
Ultrasound technology is the established first-line imaging technique for the prenatal screening and diagnosis of Placenta Accreta Spectrum. Its widespread availability and non-invasive nature make it the preferred method for assessing placental implantation. Screening is typically performed in the second or third trimester, especially for women with risk factors like a history of previous cesarean sections combined with a low-lying placenta or placenta previa.
The examination often begins with a transabdominal approach to get a general view of the placenta’s location and any obvious abnormalities. This initial scan is then supplemented by a transvaginal ultrasound, particularly to achieve a clearer, high-resolution view of the lower uterine segment where the placental invasion is most likely to occur. A full bladder is sometimes requested during the transabdominal scan to optimize visualization of the interface between the uterus and the bladder wall.
Doppler ultrasound is an important adjunct to the standard grayscale imaging. This technique helps to visualize the vascular patterns at the uterine-placental interface, which can reveal the abnormal blood vessel networks characteristic of PAS. The combination of grayscale and Doppler images allows sonographers to systematically evaluate the markers that suggest a deep placental attachment.
Key Diagnostic Indicators on Ultrasound
Sonographers and radiologists look for several markers on the ultrasound image to diagnose Placenta Accreta Spectrum. One primary indicator is the loss of the “clear zone,” the normal hypoechoic space visible between the placental tissue and the muscular uterine wall. When the placenta is abnormally attached, this dark line disappears or becomes interrupted.
Another significant finding is the presence of placental lacunae, which are irregular vascular spaces within the placental tissue itself. These spaces appear as dark, anechoic areas that often give the placenta a “moth-eaten” or “Swiss cheese” appearance on the grayscale image. The presence of three or more lacunae is highly suggestive of PAS, especially when seen in a high-risk pregnancy.
Doppler ultrasound highlights abnormal blood flow patterns. It can show hypervascularity, an excessive or chaotic network of blood vessels extending from the placenta into the uterine wall. In cases of Placenta Percreta, the ultrasound may show placental tissue bulging into the bladder cavity. This bulging often coincides with an interruption of the smooth, hyperechoic line representing the urinary bladder wall boundary, sometimes revealing “bridging vessels” crossing that boundary.
When Additional Imaging is Required
While ultrasound is highly effective, findings may be inconclusive, or the placenta’s position may make a definitive assessment difficult. For instance, a placenta located on the back wall of the uterus can be challenging to image adequately. In these ambiguous cases, or for patients with a high clinical risk factor, an additional diagnostic step is often necessary.
Magnetic Resonance Imaging (MRI) is used to provide a more detailed view of the depth of placental invasion. MRI is particularly valuable for assessing the full extent of Placenta Percreta, especially when there is a suspicion of invasion into adjacent structures like the bladder. The imaging can confirm the severity and precisely map the involvement of surrounding organs.
The optimal time for an MRI assessment is typically between 24 and 30 weeks of gestation, as imaging before this time may be less accurate. The definitive diagnosis, whether by ultrasound alone or combined with MRI, allows a multidisciplinary team to create a comprehensive management plan. This preparation ensures delivery can be performed in an appropriate medical facility with specialized surgical teams, improving maternal and neonatal outcomes.