Abnormalities of Placental Implantation (Placenta Accreta Spectrum)

Introduction

• Description: Failure of the normal process of decidua formation results in a placental implantation in which the villi directly adhere to (accreta; 79%), invade into (increta; 14%), or go through (percreta; 7%) the myometrium. One portion (partial) or all (total) of the placenta may be involved. These conditions are known as placenta accreta spectrum, formerly known as morbidly adherent placenta.

• Prevalence: Difficult to assess; estimated to be 1/272 pregnancies. The incidence has been increasing in parallel with the rate of cesarean delivery.

• Predominant Age: Reproductive age; average age is 29 years.

• Genetics: No genetic pattern.

Etiology and Pathogenesis

• Causes: Abnormal decidua formation at the time of placental implantation. Imperfect development of the fibrinoid (Nitabuch) layer. Abnormal site of placental implantation (previa, 64% of placenta accreta, cornual or lower uterine segment, or uterine scars such as site of previous cesarean delivery or myomectomy).

• Risk Factors: Placenta previa (5% without previous uterine surgery; 15%–70% with previous surgery), previous cesarean delivery (risk increases with number; 0.3% in women with one previous cesarean delivery to 6.7% for women with ≥5 cesarean deliveries), multigravidity (1/500,000 for parity <3; 1/2500 for parity >6), older maternal age (>35 years), previous uterine curettage, Asherman syndrome, previous uterine sepsis, previous manual removal of the placenta, leiomyomata, uterine malformation, prior abortion, endometrial ablation. Eighty percent of patients have a history of a uterine surgical procedure.

Signs and Symptoms

• Failure of normal placental separation

• Abnormally heavy bleeding after delivery of the placenta (may be life threatening)

• History of antepartum hemorrhage

Diagnostic Approach