Definitions

Placenta previa exists when the placenta is inserted wholly or partly into the lower segment of the uterus, partially or completely covering the internal cervical os. In cases of placenta previa, there should be a high index of suspicion for placenta accreta. Placental attachment disorders (PADs) encompass a spectrum of conditions characterized by abnormal adherence of the placenta to the uterine wall, with invasion of the placental villi through the decidua. Depending on the depth of invasion, it can be divided into three categories (accreta, increta, and percreta). PAD is commonly associated with placenta previa^{1, 2} (Figures 21.1 and 21.2).

Figure 21.1 Placenta previa

Figure 21.2 Placenta accreta (chorionic villi attach to the myometrium rather than being restricted within the decidua basalis), increta (chorionic villi invade into the myometrium), and percreta (chorionic villi invade through the myometrium)

Epidemiology, Risk Factors, Pathophysiology

Over the last 30 years, the incidence of PAD has increased 10-fold. In developed countries, reported rates range from 1 in 530 to 1 in 2,500 deliveries.³ The most common risk factors for PAD are placenta previa, previous cesarean delivery (particularly if repeated), and increasing maternal age. Less frequent risk factors include: multiparity, previous curettage, Asherman syndrome (acquired intrauterine adhesions/scarring or synechiae), and endometrial ablation. Two possible mechanisms of PAD formation have been proposed: abnormal decidualization or pathologic invasiveness of the trophoblast. The former has been considered in cases of alteration of the uterine wall, such as previous surgery, scars, and pelvic radiation,^{4, 5} whereas the latter may be explained by changes in the activity of growth and angiogenesis- and invasion-related factors within the trophoblast.⁶ Clinically, the most significant feature of placenta accreta is the abundant uteroplacental neovascularization, which can lead to life-threatening hemorrhage.⁷ Patients with PAD are often asymptomatic, but it can present with antepartum hemorrhage. If associated with placenta previa, it is a life-threatening disease and represents the second leading cause of peripartum hemorrhage, with a maternal mortality rate of 7 percent and significant morbidity due to blood loss, local organ damage, requirement for urgent hysterectomy (33–50 percent), and postoperative complications.⁸

Diagnosis

The Royal College of Obstetricians and Gynaecologists (RCOG) recommend that all pregnant women should be scanned at 20 weeks’ gestation to evaluate placental localization. If PAD is suspected, imaging should be performed at around 32 weeks’ gestation to clarify the diagnosis and allow planning for third-trimester management, including further imaging and delivery.² Women with a “low-lying placenta” (diagnosed before 28 weeks) or a placenta previa (diagnosed after 28 weeks) that is overlying a uterine scar should be reevaluated in the third trimester, with attention paid to the potential presence of PAD.⁹

The main diagnostic tools are

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  Two-dimensional (2D) gray-scale and color Doppler ultrasound

  
  
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  Three-dimensional (3D) power Doppler ultrasound

  
  
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  MRI¹⁰

According to Shih et al.,¹¹ in order to achieve the diagnosis of PAD, at least one of a number of criteria using 2D gray-scale or color Doppler ultrasound criteria should be present (Table 21.1). Three-dimensional power Doppler ultrasound can be performed as a complementary technique, as described in Table 21.1.

Figure 21.3 Loss and irregularity of retroplacental sonolucent zone

Figure 21.4 Focal lacunar flow pattern

Figure 21.5 Hypervascularity of the uterine serosa–bladder interface with abnormal blood vessels linking the placenta to the bladder

The techniques listed in the table were found to achieve a sensitivity of 92–100 percent, a specificity of 68–85 percent, and a positive predictive value of 76–88 percent.^{2, 11} MRI is no better than ultrasound, although it may be superior at detecting the depth of trophoblast invasion.¹² Dwyer et al.¹³ showed comparable sensitivity between ultrasound and MRI. The main MRI features of placenta accreta include uterine bulging, heterogeneous signal intensity within the placenta, and dark intraplacental bands on T2-weighted imaging.¹⁴ Many authors have recommended MRI for women in whom ultrasound findings are inconclusive.¹⁵ The “gold standard” for diagnosis remains pathology examination after hysterectomy characterized by the absence of decidua at the placental attachment site with the chorionic villi invading deeply into the myometrium. Intraoperative diagnosis of PAD can be made when the placenta is adherent to the uterine wall with difficulty at removal and excessive hemorrhage even with minimal placental separation.¹⁶

Planning for Delivery

If PAD is suspected, a detailed plan for follow-up, timing for delivery, multidisciplinary discussion, and patient consent is required. The discussion with the patient should include possible maternal complications, neonatal morbidity, and interventions such as interventional radiology, cell salvage during surgery, leaving the placenta in situ, and hysterectomy.² The recommended gestational age for elective cesarean delivery is usually by 36–37 weeks of gestation, before labor is likely to occur, so as to avoid emergency delivery of a woman in labor who may already be compromised by hemorrhage. Administration of maternal steroids is recommended to mature the fetal lungs and prevent transient tachypnea of the newborn, which is more likely following elective cesarean delivery before 37 weeks. Women should be encouraged to take oral iron to boost their iron stores and optimize their hemoglobin level.

Careful discussion in the case of women who refuse blood products is paramount, including documentation of which products they might consider accepting. A signed and witnessed advance decision document listing the blood products and autologous procedures that are or are not acceptable to them should be filed and a copy retained by the patient during pregnancy. There is usually no objection to intraoperative cell salvage, apheresis, hemodialysis, cardiac bypass, or normovolemic hemodilution, provided that the equipment is primed with nonblood fluids. Recombinant products, such as erythropoiesis-stimulating agents (e.g., RHuEpo) and granulocyte colony-stimulating factors (e.g., G-CSF or GM-CSF) are acceptable, as are pharmacologic agents such as IV iron or tranexamic acid. Guidelines for the use of recombinant factor VII (rFVIIa) in massive obstetric hemorrhage have been developed, although current evidence to date for use of rFVIIa in postpartum hemorrhage is limited to case reports and case series with one nonrandomized study.¹⁷