KEY POINTS
- Placenta accreta occurs in approximately 1 in 2,500 deliveries.
- Risk factors include placenta previa, Asherman’s syndrome, the existence of a prior hysterotomy scar, and advanced maternal age or parity.
- Almost 50% of all cases of placenta accreta are diagnosed antepartum.
- MRI combined with ultrasound has a sensitivity of 100% in identifying placenta accreta.
- Medical management should be considered only when the patient wishes to preserve her fertility and when no active uterine bleeding is present.
- Gravid hysterectomy has been associated with a mortality rate of 7.4%, with a 90% incidence of transfusion, a 28% incidence of postoperative infection, and a 5% incidence of ureteral injuries or fistula formation.
Placenta accreta is an uncommon but potentially lethal complication of pregnancy. It occurs when the placenta is abnormally adherent to the uterine myometrium as a result of partial or complete absence of the decidua basalis and Nitabuch’s layer. The depth of invasion determines the histologic classification: Placenta accreta indicates direct attachment of the placenta to the myometrium; placenta increta describes placental invasion into the myometrium; and placenta percreta indicates full-thickness compromise of the myometrial layer. Deeper invasion is associated with more serious complications.
Incidence and pathophysiology
The incidence of placenta accreta has increased threefold over the past 20 years. Breen and colleagues reported a rate of 1 in 7,000 deliveries in 1977,1 while a later review suggests an incidence closer to 1 in 2,500 deliveries for the period from January 1985 through December 1994.2
Placenta accreta can develop in any setting in which there is an abnormally thin or denuded decidual layer, allowing easy access to the underlying myometrium by the invading trophoblastic tissue. Risk factors include placenta previa, Asherman’s syndrome, the existence of a prior hysterotomy scar, and advanced maternal age or parity. The major contributor to the rise in the incidence of placenta accreta appears to be a concurrent increase in the rate of cesarean section, which is associated with an increased risk for placenta previa.3,4
When placenta accreta occurs in the setting of a prior hysterotomy, the placenta is implanted over the uterine scar, where the decidual layer is already thinned. Clark et al reported the association between placenta accreta and prior cesarean section in a retrospective review of over 97,000 deliveries. They discovered a 5% risk of clinically diagnosed placenta accreta with placenta previa alone, but found this risk increased to 24% with a single prior hysterotomy, to 47% with 2 prior hysterotomies, and to 67% with 3 or more (TABLE 1).3 Miller and colleagues recently demonstrated that women with placenta previa have a 9.3% incidence of placenta accreta, compared with a 0.005% incidence in women with normally located placentae.2
TABLE 1
Incidence of placenta accreta in women with placenta previa and prior hysterotomy
| NUMBER OF HYSTEROTOMIES | INCIDENCE |
|---|---|
| 0 | 5% |
| 1 | 24% |
| 2 | 47% |
| 3 or more | 67% |
Diagnosis
In the past, diagnosis was typically made clinically, suggested by significant postpartum hemorrhage or a placenta that did not separate easily from its uterine attachment. The result was treatment in an emergent setting at the time of delivery. Today, thanks to a better understanding of risk factors and improved diagnostic testing, nearly half of all cases of placenta accreta are diagnosed antepartum.5 Earlier diagnosis makes it possible for the clinician to prepare in advance for delivery and its potential complications, thus improving the ultimate outcome.
Prenatal diagnosis. The assessment of placental morphology and location is a standard part of the obstetric ultrasound examination, allowing many cases of abnormal placentation to be diagnosed antenatally. Ultrasonographic diagnostic criteria (TABLE 2) for placenta accreta include the following:
- thinning or loss of the hypoechoic retroplacental myometrial zone to less than 2 mm6,7;
- absence of the hypoechoic myometrium in the lower uterine segment between the placenta and bladder6;
- thinning or disruption of the hyperechoic uterine serosa-to-bladder interface6;
- focal exophytic masses or extension of the placenta beyond the myometrial boundaries6,7;and
- • lacunar flow within the placenta with prominent venous lakes.8
While these findings are not definitive, they are highly suggestive of the diagnosis. Most authors agree that ultrasound has a sensitivity and specificity exceeding 85% in the detection of this condition.6,9 Transvaginal studies may be preferable to transabdominal ultrasound for improved resolution. In addition, Doppler velocimetry may allow for better identification of venous lakes and areas of increased vascularity within the myometrium. The sonographic detection rate is reduced when the placenta is located posteriorly.
In cases where ultrasound is equivocal, magnetic resonance imaging (MRI) is a useful adjunct. MRI provides better delineation of tissue planes, including the placenta, myometrium, and vasculature. Kay reported 3 cases where MRI was used to identify placenta previa when ultrasonic findings were equivocal.10 Similarly, Levine et al demonstrated a sensitivity of 100% for the identification of placenta accreta using MRI with ultrasound,9 and Thorp and colleagues demonstrated the efficacy of MRI in delineating bladder involvement in a case of placenta percreta.11 As would be expected, MRI has proved most useful when the placenta is located posteriorly. Besides being safe for both mother and fetus, MRI requires little in the way of preparation. Unfortunately, it lacks portability and is more expensive to perform than ultrasound.