The placenta is an organ that develops during pregnancy to provide oxygen and nutrients to the growing fetus. Placenta Accreta Spectrum (PAS) is a serious condition where the placenta attaches abnormally deep into the wall of the uterus, failing to separate properly after childbirth. This inability to detach can lead to severe hemorrhage, a major complication for the mother. Ultrasound technology is the primary, non-invasive method utilized by healthcare providers to assess the depth of placental attachment and screen for this condition.
Understanding Placenta Accreta Spectrum
Placenta Accreta Spectrum is defined by the depth of placental tissue invasion into the myometrium, the muscular layer of the uterine wall.
The least invasive form is Placenta Accreta, where the placenta’s villi attach firmly to the myometrium without penetrating it. This is the most common form, accounting for approximately 75% of all cases.
The next level is Placenta Increta, where the placental villi invade into the myometrium, embedding deep within the muscle. The most severe form is Placenta Percreta, where the tissue grows through the entire uterine wall and may invade nearby organs, such as the bladder or rectum. The level of invasion determines the complexity of clinical management and potential complications.
Ultrasound Screening: Why and When It’s Used
Screening for PAS is not routinely performed but is targeted toward women with specific risk factors. The necessity for screening is driven by a history of previous uterine surgeries, as these procedures can damage the lining of the uterus. Prior cesarean delivery is the most significant risk factor, and the risk of PAS increases with each subsequent section.
Other risk factors include placenta previa (a placenta that covers the cervix) and any history of uterine curettage or other uterine surgeries. Screening typically begins with a focused ultrasound examination during the late second or early third trimester, often between 28 and 30 weeks of gestation. The goal is to identify visual markers suggesting deep placental adherence, allowing for proactive delivery planning.
Specific Sonographic Markers for Diagnosis
Accurate diagnosis of PAS depends on recognizing a distinct set of visual markers on the ultrasound image. One primary sign is the presence of placental lacunae, which are irregular, hypoechoic (darker) vascular spaces within the placental tissue. These spaces give the placenta a characteristic “Swiss cheese” appearance; having more than three lacunae is highly suggestive of the condition.
Another marker is the loss of the “clear zone,” the normal hypoechoic space between the placenta and the uterine muscle. When the placenta attaches abnormally, this clear space becomes irregular or disappears entirely, indicating direct attachment to the myometrium. Additionally, the myometrium itself may appear markedly thin, sometimes less than 1 millimeter, especially near a prior cesarean scar.
Color Doppler imaging is employed to assess blood flow patterns where the placenta meets the uterus. Abnormal Doppler findings include hypervascularity (an increase in blood vessels) at the subplacental level or the uterovesical interface (the boundary between the uterus and the bladder). Visualization of large vessels bridging across the uterine-placental margin further raises suspicion for PAS.
The Diagnostic Pathway: Confirmation and Specialized Care
A suspicious ultrasound finding initiates a pathway toward further confirmation and specialized clinical planning. While ultrasound is the primary diagnostic tool, Magnetic Resonance Imaging (MRI) is frequently used as a supplemental tool, especially in complex or inconclusive cases. MRI offers a clearer view of the depth of invasion, which is helpful when Placenta Percreta is suspected, as it can better assess if the placenta has invaded surrounding organs.
The goal of prenatal diagnosis is to ensure the mother receives the safest possible care during delivery. A confirmed or highly suspected diagnosis of PAS necessitates the transfer of care to a specialized center, often a high-level maternity hospital. This facility provides access to a multidisciplinary team (MDT), including obstetricians, gynecologic surgeons, interventional radiologists, anesthesiologists, and neonatologists. This team collaborates to plan a controlled delivery, often involving a scheduled cesarean section followed by a hysterectomy to manage life-threatening bleeding.