Review the relevance of diagnosing Placenta Accreta Spectrum (PAS).
Discuss first trimester markers for PAS.
Examine second and third trimester markers for PAS.
Review best practices for optimizing imaging in PAS.
Importance of Diagnosing Placenta Accreta Spectrum
Rising Incidence of PAS:
In the 1970s, the rate of PAS was reported at 1 in 3,000. This has increased to rates as high as 1 in 272 in recent series.
Maternal Morbidity Risk:
PAS is associated with an eighteen-fold increased risk of severe maternal morbidity.
There is a forty times increased risk of ICU admission associated with PAS.
Prenatal Diagnosis and Care:
Prenatal diagnosis paired with multidisciplinary team care can improve outcomes significantly for patients with PAS.
Risk Assessment Data
Data from the Maternal Fetal Medicine Unit's network indicates that:
In cases with prior cesarean sections, the risk of PAS does not significantly increase if there is no placenta previa.
However, if there is a placenta previa with prior cesarean deliveries, the risk of PAS may approach 70% in patients with five prior cesarean sections.
A task force led by Doctor Muhammad was established to standardize the definitions and approaches for ultrasound examinations in pregnancies at risk of PAS.
First Trimester Markers of Placenta Accreta Spectrum
Lower Uterine Segment Implantation:
A crucial marker for PAS is finding implantation in the lower uterine segment during the first trimester.
When analyzing a midsagittal plane of the uterus during a dating ultrasound, an implantation in the lower half of the uterus (particularly the anterior lower quadrant) is considered low implantation.
Cesarean Scar Ectopic Pregnancies:
Cesarean scar ectopic pregnancies share features with PAS, characterized by abnormal implantation of trophoblasts on or in the scar tissue of a previous cesarean section.
Incidence of cesarean scar ectopic pregnancy is reported to range from 1 in 1,800 to 1 in 2,600.
In a study by Kallie et al. (2018), 75% of cesarean scar ectopic pregnancies had surgical or pathological evidence of PAS, with two-thirds being confirmed as placenta previa.
Early diagnosis of cesarean scar ectopic pregnancies is most effective when detected before 8 weeks gestation.
Types of Cesarean Scar Ectopic Pregnancies:
Exogenic: Implantation in the niche of the cesarean scar.
Endogenic: Implantation on the scar; often characterized by a tethered gestational sac.
Management:
Recommended management for cesarean scar ectopic pregnancies includes expectant management, intragestational injection of methotrexate (with or without balloon compression), or operative resection.
Versus Normal Pregnancy:
It is important to note that not all lower uterine segment implantation results in PAS. Some pregnancies, such as those with a gestational sac in the middle segment of the uterine cavity, are normal.
Technical Notes:
The gestational sac often grows towards areas of least resistance, typically the uterine cavity.
Second Trimester Markers of Placenta Accreta Spectrum
Comparison of Markers:
A study comparing PAS cases to those without PAS indicated that:
The number and size of placenta lacunae, as well as their characteristics in grayscale and color Doppler, were significantly higher in PAS cases.
Abnormalities in the uteroplacental interface were observed in 85% of PAS cases.
Presence of Lacunae:
Identification of anechoic spaces consistent with lacunae through first trimester ultrasounds is vital.
Increased vascularity and the presence of large lacunae in PAS cases can be observed as early as 11 weeks gestation.
Marker Identification Examples:
Presence of significant vascularity and abnormal placental borders observable through ultrasound techniques.
Trends Over Gestation:
Many second trimester markers can be observed as early as the first trimester, increasing the importance of early ultrasound evaluations.
Imaging Techniques for PAS
Sensitivity and Specificity:
A meta-analysis revealed that first trimester ultrasounds have a sensitivity of 86%, specificity of 63%, and an AUC of 0.83 for diagnosing PAS.
Ultrasound Markers in Later Trimesters:
Increased velocity or turbulent blood flow on color Doppler imaging often correlates with PAS.
Multiple lacunae can predict the presence of PAS, particularly in patients with a history of cesarean section and previa.
Normal Placenta Analysis:
Normal findings show clear retroplacental spaces; however, high numbers of lacunae could indicate the risk of PAS.
Striking color Doppler may falsely indicate increased vascularity; careful assessment is required to avoid misdiagnosis.
Technical Recommendations:
Use a transvaginal approach to fully assess the placenta as a three-dimensional organ; pressure on the abdomen must be minimized during examination.
Maintain settings on color Doppler so that low velocities, low wall filters, and high gains are used to avoid misdiagnosing vascularity.
Conclusion and Takeaways
High-risk factors include the presence of a prior cesarean section coupled with a uterus that shows previa.
A lower uterine segment's thickness can be a significant marker for PAS.
Ongoing development of localized protocols for ultrasound evaluation is recommended to ensure consistency across assessments.
Assessing the gestational sac's location relative to the lower uterine segment and surgical history is critical.
Regular ultrasound reporting is suggested, categorizing risk as low, intermediate, or high based on findings relevant to PAS.
Final Remarks
Emphasis on early detection and appropriate referral for better management of PAS cases. A systematic approach with clear protocols can lead to improved patient outcomes.