Placenta accreta is an uncommon but important complication of pregnancy and is associated with high maternal and fetal mortality and morbidity. Placenta accreta occurs when the placental implantation is abnormal: the decidua basalis that normally separates the anchoring villi and the myometrium is missing The placenta does not separate properly from the uterus after delivery, which gives rise to maternal hemorrhage.
Placenta accreta ranges in severity from mild forms, which are only visible microscopically, to severe, grossly apparent forms where invasion extends into adjacent organs.
• Placenta accreta: is not only the general term for all forms of this disorder, but also refers to the least invasive subtype, where placental villus invasion is confined to the myometrial surface
The more severe subtypes are:
• Placenta increta: where trophoblastic tissue invades deeper into the myometrium.
• Placenta percreta: where trophoblastic tissue invasion extends through the myometrium to the serosal surface of the uterus and sometimes beyond (Rosai, 2004).
Placenta accreta is much more common than placenta increta and percreta. In a pooled analysis of results from two series involving a total of 138 histologically confirmed, abnormally implanted placentas from hysterectomy specimens, the type and frequency of abnormal placentation were (Wu et al., 2005):
■ Placenta accreta: 79 percent
■ Placenta increta: 14 percent
■ Placenta percreta: 7 percent
Placenta accreta remains the most common reason for Caesarean hysterectomy in developed countries (Shellhaas et al., 2009; Flood et al., 2009)
INCIDENCE:
The incidence of placenta accreta appears to be increasing.
o In 1950s, placenta accreta was rare, occurring in 1 in 30, 000 deliveries.
o In 1970s, the incidence was estimated to be 1 in 4, 027 deliveries.
o In 1980s, the incidence increased to be 1 in 2, 510 (Read et al., 1980).
o For the period of 1982-2002, researchers have reported the incidence of placenta accreta as 1 in 533 deliveries (Wu et al., 2005).
o In 2006, the incidence increased to be 1 in 210 deliveries (Stafford and Belfort, 2008).
o Recently, a national case – control study using the UK obstetric surveillance system was undertaken, including 134 women diagnosed with placenta accrete/increta/percreta between May 2010 and April 2011 and 256 control women.
The estimated incidence of placenta accreta/increta/percreta was 1.7 per 10, 000 maternaty overall;577 per 10, 000 in women with both a previous cesarean section and placenta previa (Fitzpatrick et al., 2012).
The marked increase in incidence has been attributed to the increasing prevalence of cesarean delivery in recent years.
PATHOGENESIS:
The pathogenesis of placenta accreta is not known with certainty. The most common theory is that:
Defective decidualization (thin, poorly formed, or absent decidua) related to previous surgery or to anatomical factors (endocervix, lower uterine segment, endosalpinx, uterine anomaly) allows the placenta to attach directly to the myometrium (Tantbirojn et al., 2008; Khong, 2008). This theory is supported by the observation that 80 percent of these cases are associated with a history of previous cesarean delivery, curettage, and/or myomectomy (Tantbirojn et al., 2008). Other theories, which may account for a proportion of cases, attribute placenta accreta to excessive extravillous trophoblastic invasion or defective maternal vascular remodeling in the area of a hysterotomy scar (Jauniaux and Jurkovic, 2012).
The occurrence of placenta increta and percreta may be due to partial or complete dehiscence of a uterine scar, which allows the extravillous trophoblast direct access to the deeper myometrium, serosa, and beyond (Tantbirojn et al., 2008).
RISK FACTORS:
The most important risk factor for placenta accreta is placenta previa after a prior cesarean delivery. In women with placenta previa, the frequency of placenta accreta increases with an increasing number of cesarean deliveries as follows (Silver et al., 2006):
■ No previous cesarean birth, 1 to 5 percent
■ One previous cesarean birth, 11 to 25 percent
■ Two previous cesarean births, 35 to 47 percent
■ Three previous cesarean births, 40 percent
■ Four or more previous cesarean births, 50 to 67 percent
In the absence of placenta previa, the frequency of placenta accreta still increases with an increasing number of cesarean deliveries, but the incidence is much lower. In women without placenta previa, the frequency of placenta accreta is (National Institutes of Health Consensus Development Conference Statement, 2010):
■ One previous cesarean birth, 0.3 percent
■ Two previous cesarean births, 0.6 percent
■ Three previous cesarean births, 2.4 percent
Other risk factors for placenta accreta include uterine instrumentation and intrauterine scarring, all of which may be associated with damage to or absence of the decidua basalis, as well as smoking, maternal age over 35, grand multiparity and recurrent miscarriage (ACOG, 2002; Gielchinsky et al., 2002).
In addition, an association was found between placenta accreta ( PA ) and abnormally elevated second-trimester alpha fetal protein (AFP) and free subunit of human chorionic gonadotrophin (-hCG) levels in maternal serum (Usta et al., 2005; Wu et al., 2005).
Clinical presentation:
Ideally, placenta accreta is first suspected because of findings on obstetrical ultrasound examination while the patient is asymptomatic. The abnormally implanted placenta is often diagnosed on prenatal sonographic evaluation of the placenta in a woman with risk factors for accreta (previa, previous cesarean delivery), but may be an incidental finding.
The first clinical manifestation of placenta accreta is usually profuse, life-threatening hemorrhage that occurs at the time of attempted manual placental separation. Part, or all, of the placenta remains strongly attached to the uterine cavity, and no plane of separation can be developed.
Sequelae:
Poorly controlled hemorrhage related to placenta accreta, increta and percreta is the indication for one to two thirds of peripartum hysterectomies (Glaze et al., 2008). Additional potential sequelae of massive hemorrhage include disseminated intravascular coagulopathy, adult respiratory distress syndrome, renal failure, unplanned surgery, and death.
Other complications include intraabdominal infection, bladder injury, ureteral damage, bowel injery, and fistula formation
A case-control study including 310 patients with placenta accreta reported a significant increase in preterm birth (11 versus 1 percent in controls) and small for gestational age infants (27 versus 14 percent in controls) Neonatal outcome does not appear to be significantly affected by depth of placental invasion
Mortality rates as high as 7% are reported to be associated with placenta accreta as well as the additional potential intraoperative/post operative morbidity assaciated with massive blood transfusion, hypovolemai, and infection
Given the significant morbidity associated with placenta accreta, Prenatal diagnosis of is critical to obstetric outcome, by enabling early arrangements for elective caesarean section and possible hysterectomy, and decreases placenta accreta-associated complications such as the need for the transfusion of blood products
It is likely that antenatal diagnosis of placenta accreta has contributed to the overall drop in maternal morbidity and deaths that has been associated with this condition (Eller et al., 2009; Stafford and Belfort, 2008).
Physicians must maintain a high level of suspicion, particularly in patients with risk factors such as multiparity (especially previous cesarean section deliveries) in association with placenta previa
Antenatal diagnosis of placenta accreta has major advantages that include the ability to plan all aspects of care. In addition, particularly high-risk cases (cervical invasion, bladder involvement) may potentially be identified. A standardized protocol can be instituted that includes the involvement of multiple disciplines (maternal-fetal medicine, neonatology, anesthesiology, nursing, interventional radiology, blood bank, gynecologic oncology), prophylactic placement of arterial catheters and/or ureteral stents if desired, and electively operating at a chosen gestational age with an assembled surgical team and equipment
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