Placenta accreta spectrum (PAS) describes abnormal invasion of placental tissue into or through the myometrium, comprising 3 distinct conditions: placenta accreta, placenta increta, and placenta percreta. This complication is relatively new to obstetrics, first described in 1937.1
The overall incidence of PAS has been increasing over several decades, in parallel to an increasing rate of cesarean delivery (CD), with an incidence from 1982 through 2002 of 1 in 533 pregnancies, representing a 5-fold increase since the 1980s.2 PAS is associated with significant morbidity and mortality, including fetal growth restriction, preterm delivery, placental abruption antenatally, and hemorrhage during delivery or postpartum.
Prenatal diagnosis of PAS and planned delivery at an experienced center are associated with significant reduction in maternal and fetal morbidity.3 In an era of advanced imaging modalities, prenatal detection of PAS regrettably remains variable and largely subjective: As many as 20% to 50% of cases of PAS escape prenatal diagnosis.3,4
In this article, we review the sonographic markers of PAS, including diagnostic accuracy, and propose a standardized approach to prenatal diagnosis. Throughout our discussion, we describe protocols for detection of PAS practiced at our Maternal-Fetal Medicine Program in the Department of Obstetrics and Gynecology, Eastern Virginia Medical School (also see “US evaluation of PAS risk: The authors’ recommended approach”).
Numerous risk factors
There are many risk factors for PAS, including prior uterine surgery or instrumentation, such as CD, uterine curettage, myomectomy, pelvic radiation, and endometrial ablation. Other risk factors include smoking, in vitro fertilization, advanced maternal age, multiparity, and a brief interval between prior CD and subsequent pregnancy.5 Of major significance is the increased risk of PAS in the presence of placenta previa with prior CD.6 Knowledge of clinical risk factors by the interpreting physician appears to be associated with improved detection of PAS on ultrasonography (US).4
Ultrasonographic markers of PAS
Sonographic markers of PAS in the first trimester include:
- a gestational sac implanted in the lower uterine segment or in a CD scar
- multiple hypoechoic spaces within the placenta (lacunae).7
Lower uterine-segment implantation has been defined by Ballas and colleagues as 1) a gestational sac implanted in the lower one-third of the uterus between 8 and 10 weeks’ gestation or 2) a gestational sac occupying primarily the lower uterine segment from 10 weeks’ gestation onward (FIGURE 1).8 Our experience is that it is difficult to accurately assess lower uterine-segment implantation beyond 13 weeks of gestation because the sac typically expands to fill the upper uterine cavity.
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