Placenta accreta is both the general term applied to abnormal placental adherence and also the condition seen at the milder end of the spectrum of abnormal placental adherence. This article focuses on the second, more specific definition.
In a placenta accreta, the placental villi extend beyond the confines of the endometrium and attach to the superficial aspect of the myometrium but without deep invasion.
It is the most common form of placental invasion (~75% of cases). It is thought to occur in approximately 1 in 7,000 pregnancies. The incidence is increasing due to the increased practice of Cesarean sections. The combination of previous Cesarean section and an anterior placenta previa should raise the possibility of a placenta accreta. This disease has a maternal mortality of up to 7% depending on location.
Recognised primary risk factors for placenta accreta include:
- placenta praevia
- prior Cesarean section
- uterine anomalies
- previous uterine surgery
- dilation and curettage
- maternal age greater than 35 years
The abnormal implantation is thought to result from a deficiency in the decidua basalis, in which the decidua is partially or completely replaced by loose connective tissue. In a placenta accreta, chorionic villi and/or cytotrophoblasts directly attach to the myometrium with little or no intervening decidua.
Accurate prenatal diagnosis of placenta accreta is vital because this abnormality is an important cause of significant hemorrhage in the immediate post-delivery period with resultant maternal and fetal morbidity and mortality. However, the diagnosis is not often made prospectively.
According to one study 9, ultrasound has a sensitivity of 89.5%, the positive predictive value of 68% and negative predictive value of 98% for the diagnosis of placenta accreta.
Several sonographic criteria for the diagnosis of placenta accreta have been reported:
- marked thinning or loss of the retroplacental hypoechoic zone
- interruption of the hyperechoic border between the uterine serosa and bladder
- presence of mass-like tissue with echogenicity similar to that of the placenta
- visualization of prominent vessels or lakes within the placenta or myometrium
- visualization of lacunae has the highest sensitivity in the diagnosis of PA, allowing identification in 78%-93% of cases after 15 weeks gestation, with a specificity of 78.6%10
When a placenta accreta occurs on the posterior or lateral walls of the uterus, it may be difficult to detect by ultrasound.
Magnetic resonance imaging has also been used to diagnose placenta accreta. Specific fast acquisition sequences (e.g. HASTE, true FISP) help to minimise fetal and maternal motion artefacts.
The demonstration of uterine bulging and loss of normal uterine contour.
- on T2 weighted MR images, the mass is hyperintense and may be heterogeneous
- also, T2 weighted MR images are useful in the assessment of focal thinning of the myometrium and interruption of the junctional zone
Treatment and prognosis
A definitive treatment for placenta accreta consists of a hysterectomy with possible resection of adjacent organs if percreta is present. A placenta accreta is reported to be the most common indication for emergency peripartum hysterectomy.
In certain instances, however, conservative treatment may be used, especially if uterine preservation is desired. Conservative measures include curettage, oversewing of the placental bed, and ligation of the uterine arteries or the anterior divisions of the internal iliac arteries.
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