Placenta accreta spectrum (PAS) disorder represents a significant and growing challenge in modern obstetrics, characterized by an abnormal adherence of the placenta to the uterine wall. This condition arises from a defect in the decidua basalis, the layer of the uterine lining that normally separates the placenta from the myometrium, leading to direct attachment or even invasion of the chorionic villi into the myometrium. The increasing incidence of PAS disorder worldwide is primarily linked to the rising rate of Cesarean sections and other uterine surgeries, rendering it a major cause of peripartum hemorrhage, obstetric hysterectomy, and maternal and neonatal morbidity and mortality. Understanding the various facets of placenta accreta—from its underlying causes and clinical manifestations to its intricate diagnostic and therapeutic pathways—is crucial for optimizing patient outcomes and ensuring the safety of mothers facing this high-risk obstetric complication.
Discussion of Placenta Accreta
Placenta accreta spectrum disorder is a condition where the placenta abnormally adheres to or invades the myometrium (the muscular wall of the uterus) rather than detaching normally after childbirth. This abnormal adherence is due to a partial or complete absence of the decidua basalis, the physiological barrier between the placenta and the uterine wall. The severity of placental invasion is categorized into three main types:
- Placenta Accreta (vera): The most common form, where the chorionic villi directly attach to the myometrium without invading it.
- Placenta Increta: A more severe form where the chorionic villi invade into the myometrium itself.
- Placenta Percreta: The most severe and least common form, where the chorionic villi penetrate through the entire thickness of the myometrium and serosa, potentially invading adjacent organs such as the bladder, bowel, or major pelvic vessels.
The clinical significance of PAS disorder lies in its potential for catastrophic hemorrhage during attempts to manually remove the placenta after delivery. Normal placental separation involves the cleavage of the decidual layer; however, in PAS, this cleavage plane is absent, leading to massive bleeding, need for blood transfusions, prolonged hospitalization, hysterectomy, and in severe cases, maternal death. The increasing global incidence of PAS disorder, estimated to be as high as 1 in 500 pregnancies in some populations, poses a substantial burden on healthcare systems and necessitates a heightened awareness and specialized approach to management. This rising trend is closely associated with changes in obstetric practices, particularly the increased rate of Cesarean deliveries over the past few decades.
Etiology of Placenta Accreta
The primary driver and most significant risk factor for placenta accreta spectrum disorder is a history of previous Cesarean section, especially when combined with placenta previa. The mechanism involves a scarred uterine wall from prior surgeries creating an area where the decidual layer is deficient or absent. When a subsequent pregnancy implants over this scarred area, the trophoblast (the outer layer of the blastocyst that forms part of the placenta) can invade directly into the myometrium. The risk of PAS increases with the number of previous Cesarean sections:
- One prior C-section with placenta previa: 3% risk
- Two prior C-sections with placenta previa: 11% risk
- Three prior C-sections with placenta previa: 40% risk
- Four or more prior C-sections with placenta previa: Over 60% risk
Placenta previa itself is a major independent risk factor because it signifies placental implantation in the lower uterine segment, an area often thinned and more prone to decidual defects, especially after prior uterine surgery. Other contributing risk factors include:
- Previous uterine surgery: Myomectomy (surgical removal of fibroids), uterine curettage (D&C), hysteroscopy, or endometrial ablation can also lead to uterine scarring and decidual defects.
- Advanced maternal age: Women over 35 years old have an increased risk.
- Multiparity: A higher number of previous pregnancies.
- Assisted reproductive technologies (ART): Pregnancies conceived via ART have a slightly elevated risk, possibly due to altered endometrial receptivity or higher rates of multiple gestations.
- Uterine anomalies: Conditions like septate uterus.
- Hypertension and smoking: Some studies suggest a potential, though less significant, association.
The fundamental etiologic pathway involves a defect or absence of the Nitabuch’s layer and spongy layer of the decidua basalis, which normally acts as a barrier to trophoblast invasion. This allows the chorionic villi to anchor more deeply than usual into the uterine wall, leading to the various degrees of abnormal adherence and invasion characterizing PAS disorder.
Symptoms and Signs of Placenta Accreta
One of the most challenging aspects of placenta accreta spectrum disorder is its typically asymptomatic nature throughout the majority of pregnancy. Most women with PAS disorder will have an unremarkable prenatal course without any specific symptoms or signs directly attributable to the condition itself. The primary clinical manifestation and the most dramatic sign of placenta accreta occurs post-delivery, during the third stage of labor.
Normally, after the baby is born, the placenta spontaneously detaches from the uterine wall within a few minutes, facilitated by uterine contractions. In cases of PAS disorder, the placenta remains firmly adhered to the myometrium. When an attempt is made to manually remove the placenta, or if spontaneous detachment fails, it results in:
- Massive and uncontrollable hemorrhage: This is the hallmark clinical sign. The bleeding can be rapid, profuse, and life-threatening, often requiring immediate blood transfusion and emergency intervention. The inability of the uterus to contract effectively after delivery (due to the embedded placenta) further exacerbates blood loss.
- Failure of placental separation: The placenta either does not separate at all, or only partially separates, leaving fragments adhered.
While most cases are silent antenatally, in very severe forms, particularly Placenta Percreta, some subtle signs may emerge late in pregnancy if adjacent organs are invaded:
- Hematuria: Blood in the urine, if the bladder is invaded. This can be macroscopic (visible) or microscopic.
- Abdominal pain or discomfort: Rarely, if the percreta extends extensively or causes localized irritation outside the uterus.
- Rectal bleeding: Extremely rare, if bowel invasion occurs.
The lack of distinct antenatal symptoms underscores the critical importance of early and accurate antenatal diagnosis through imaging, particularly in high-risk pregnancies. Without antenatal diagnosis, the condition is often discovered acutely during delivery when severe hemorrhage ensues, leading to emergency situations with less optimal outcomes due to lack of preparedness.
Diagnosis of Placenta Accreta
The timely and accurate antenatal diagnosis of placenta accreta spectrum disorder is paramount for optimal maternal and neonatal outcomes, allowing for planned delivery in a specialized facility with a multidisciplinary team.
1. Antenatal Ultrasound: This is the primary and most effective diagnostic modality. A targeted ultrasound by an experienced sonographer, particularly in high-risk patients (e.g., those with placenta previa and prior C-sections), is crucial. Key sonographic signs suggestive of PAS include:
- Loss of the clear retroplacental zone (clear zone): Normally, a hypoechoic space exists between the placenta and the myometrium. Its absence or irregularity is a significant indicator.
- Presence of placental lacunae (Swiss cheese appearance): These are large, irregular vascular spaces within the placenta that represent dilated uteroplacental vessels. They are highly specific for PAS.
- Thinning or disruption of the myometrial-bladder interface: In cases of percreta invading the bladder, the normal hyperechoic line separating the uterus from the bladder may be interrupted.
- Increased vascularity and turbulent flow at the uteroplacental interface on Doppler imaging: Color Doppler can demonstrate abnormal vascularity extending into the myometrium.
- Extension of placental tissue into the uterine serosa or beyond: Direct visualization of placental tissue invading the uterine outer layer or adjacent organs.
Ultrasound should ideally be performed in the second or third trimester, typically between 18-24 weeks and again around 32-34 weeks, especially for high-risk cases.
2. Magnetic Resonance Imaging (MRI): MRI serves as an adjunct to ultrasound, particularly in cases where ultrasound findings are equivocal, or for posterior placentas where visualization is challenging, or when there is strong suspicion of placenta percreta with bladder or bowel invasion. MRI provides a wider field of view and superior soft tissue contrast, which helps in:
- Confirming the depth of invasion: Differentiating accreta, increta, and percreta more precisely.
- Assessing invasion of adjacent organs: Clearly delineating involvement of the bladder, bowel, or parametrial tissues.
- Identifying abnormal placental signal intensity and heterogeneity.
- Visualizing dark intraplacental bands on T2-weighted images.
MRI is typically performed in the late second or early third trimester. While highly valuable, it is more expensive and less readily available than ultrasound.
3. Other Diagnostic Considerations:
- Clinical suspicion: Any patient with placenta previa and a history of prior Cesarean sections should be considered at high risk and undergo thorough imaging surveillance.
- Biomarkers: While research is ongoing, no reliable serum biomarkers for PAS are currently used in routine clinical practice.
- Histopathology: The definitive diagnosis of placenta accreta spectrum disorder is made retrospectively through histopathological examination of the hysterectomy specimen, confirming the absence of decidua and invasion of chorionic villi into the myometrium.
A multidisciplinary approach involving obstetricians, radiologists, and maternal-fetal medicine specialists is crucial for accurate antenatal diagnosis and subsequent management planning.
Treatment of Placenta Accreta
The management of placenta accreta spectrum disorder requires a highly specialized, multidisciplinary approach in a tertiary care center equipped to handle massive obstetric hemorrhage. The primary goal is to minimize maternal morbidity and mortality.
1. Pre-delivery Planning and Multidisciplinary Team:
- Planned Delivery: For antenatally diagnosed cases, an elective delivery is typically scheduled between 34 and 36 weeks of gestation. This timing aims to balance fetal maturity with the increasing risk of spontaneous hemorrhage or preterm labor as the pregnancy progresses.
- Multidisciplinary Team: A dedicated team is essential, including maternal-fetal medicine specialists, obstetric anesthesiologists, surgical oncologists/gynecologic oncologists (for complex percreta cases), interventional radiologists, urologists (if bladder invasion is suspected), neonatologists, and a blood bank team.
- Blood Product Availability: Adequate blood products (packed red blood cells, fresh frozen plasma, platelets, cryoprecipitate) must be on standby and immediately available.
- Informed Consent: Thorough counseling regarding the risks, potential for hysterectomy, and alternative management strategies is crucial.
2. Cesarean Hysterectomy:
- Standard of Care: For most cases of PAS disorder, particularly increta and percreta, the recommended approach is a planned Cesarean hysterectomy with the placenta left in situ.
- Procedure: A classical or high transverse uterine incision is made to avoid the placenta. The baby is delivered, and the umbilical cord is ligated and cut close to the placenta, leaving the placenta entirely inside the uterus. A hysterectomy is then performed without attempting to remove the placenta, as this is the maneuver most likely to cause catastrophic hemorrhage.
- Benefits: This approach minimizes blood loss, reduces the risk of infection, and decreases operative time compared to attempting piecemeal placental removal.
3. Conservative Management (Placenta Left In Situ):
- For Future Fertility: In select cases, particularly for placenta accreta vera, and if the patient strongly desires future fertility, a conservative approach of leaving the placenta in situ without hysterectomy may be considered.
- Risks: This approach carries significant risks, including delayed hemorrhage, infection, Asherman’s syndrome, and the need for subsequent interventions (e.g., methotrexate, uterine artery embolization). The success rate varies, and close monitoring for several weeks or months post-delivery is required until the placenta involutes. This option should only be pursued after extensive counseling and in highly specialized centers.
4. Adjunctive Therapies:
- Uterine Artery Embolization (UAE): May be used pre- or post-operatively, sometimes by interventional radiologists, to reduce blood flow to the uterus, either to control active hemorrhage or to aid in conservative management.
- Methotrexate: Used historically in conservative management to accelerate placental resorption, but its effectiveness and routine use are debated due to potential side effects and limited evidence of benefit in accelerating placental resolution.
5. Intraoperative Considerations:
- Cell Saver: Use of an intraoperative cell saver device to collect, clean, and reinfuse the patient’s own blood.
- Pelvic Balloon Catheters/Internal Iliac Artery Ligation: In some cases, placement of balloon catheters in the internal iliac arteries pre-operatively may be considered to temporarily occlude blood flow during the most critical parts of the surgery, though evidence for improved outcomes is mixed.
The management of placenta accreta is complex and highly individualized. While Cesarean hysterectomy remains the safest and most definitive treatment for severe cases, careful planning, experienced personnel, and access to advanced resources are crucial for reducing the profound risks associated with this increasingly prevalent obstetric complication.
References
- American College of Obstetricians and Gynecologists. (2018). Placenta Accreta Spectrum. ACOG Committee Opinion No. 767. Obstetrics & Gynecology, 133(1), e1-e7.
- Silver, R. M., & Branch, D. W. (2018). Placenta Accreta Spectrum. New England Journal of Medicine, 378(16), 1529-1536.
- Wright, J. D., Clark, E. A., & Sun, L. (2021). Placenta Accreta Spectrum: Diagnosis and Management. Obstetrics & Gynecology Clinics of North America, 48(2), 297-313.
- D’Antonio, F., Iacovella, C., & Bhide, A. (2020). Placenta accreta spectrum disorder: a structured approach to diagnosis and management. Fetal Diagnosis and Therapy, 47(4), 261-270.
- Society for Maternal-Fetal Medicine (SMFM) Publications Committee. (2018). Placenta Accreta Spectrum Disorder: ACOG Committee Opinion No. 767. American Journal of Obstetrics & Gynecology, 219(2), B2-B4.
