Placental Development, Indications for and Methods of Examination
Section 5 -
Abnormalities of Implantation
Phyllis C. Huettner, M.D.
Placenta Accreta, Increta and Percreta
Case 6: Placenta Percreta
Placenta accreta refers to a spectrum of findings with the common clinical feature of
abnormal placental adherence and the common pathologic feature of absence of decidua between chorionic
villi and myometrium. Accreta can be subdivided into placenta accreta vera where the chorionic villi
abut against but do not invade the myometrium, placenta increta where the chorionic villi extend into the
myometrium and placenta percreta where the uterus is perforated by chorionic villi that may then invade
other structures such as the bladder. Placenta accreta may be focal, partial or complete.
True incidence figures are very difficult to determine depending on whether clinical or a
histopathologic criteria are used, the decades studied and the population studied. The reported
incidence ranges from 1 in 540 to 1 in 93,000 deliveries. The incidence from a large series of accreta
confirmed histologically in hysterectomy specimens is about 1 in 2500 deliveries. This represents a 10
fold increase in incidence over the last 10 years. Partial accreta is probably more common than complete
and may be under reported.
Placenta accreta is associated with certain well-established risk factors. The two most
important of these are placenta previa and a history of prior Cesarean section. In one study, 89% of
patients with accreta had previa and 73% had a prior Cesarean section. About one in 10 women with previa
and one in 5 with both previa and prior Cesarean section develop accreta. In many cases, only that part
of the placenta implanted over a prior Cesarean section scar is abnormally adherent underscoring the
relationship between scar tissue and abnormal adherence. Other risk factors such as prior D&C and a
history of uterine sepsis may also reflect uterine scarring as a result of trauma or infection. The need
for manual removal of the placenta in a prior pregnancy may indicate a minor degree of abnormal adherence
in the prior pregnancy and increases the risk of accreta in subsequent pregnancies. Implantation in
non-fundic areas such as the cornu, over a submucosal fibroid or in a rudimentary uterine horn also
increases the risk of accreta. The risk is also increased with increasing maternal age. Otherwise
unexplained second-trimester elevated maternal serum alpha-fetoprotein and beta-hCG levels have been
associated with increased incidence of placenta accreta. In about 10% to 20% of cases none of these risk
factors are identified, prompting some to speculate that in some cases genetic factors may be
Most cases of accreta are diagnosed during labor. Nevertheless, in women with risk factors
prenatal ultrasound may be helpful. It has a sensitivity of about 65% and a specificity of about 40%.
Color Doppler is even more sensitive and specific. A recent study in women with ultrasound results
suggestive of accrete showed an improved sensitivity and specificity with pMRI. MRI was able to
accurately determine the location of abnormal adherence and depth of invasion, factors that determine the
likelihood of significant bleeding and may modify the surgical approach and degree of clinical readiness
Clinical symptoms are usually not apparent and the diagnosis is often not suspected until
after delivery of the baby when there is difficulty delivering the placenta. Placenta accreta is
completely compatible with normal intrauterine growth and development although a recent study reports an
increase in preterm delivery and delivery of small for gestational age infants with accreta. A minority
of patients report severe abdominal pain and tenderness during pregnancy with no other explanation. All
degrees of placenta accreta may be associated with severe postpartum hemorrhage. Over half of patients
with accreta require transfusion; one in five requires 5 or more units. Often the only way to stop the
bleeding is to perform an immediate hysterectomy. In large series of Cesarean and partum hysterectomy,
accreta accounts for about a third of such hysterectomies. About 30% of patients experience antepartum
bleeding but nearly all of these also have placenta previa. Uterine rupture complicates 13.8% of cases.
Most ruptures occur in the second trimester prior to the onset of labor, almost always at the site of a
previous scar. Uterine inversion is uncommon, complicating 2.1% of cases. It usually follows repeated
attempts at manual extraction. Maternal mortality rate is 7% and is usually due to shock or DIC from
extensive hemorrhage. Fetal death occurs in about 1% of cases and is usually related to uterine rupture
and severe antepartum bleeding.
Clinical management depends on the degree of hemorrhage. In one early study, the mortality
rate in women treated by immediate hysterectomy was four times lower than that in woman initially treated
by conservative management. More recently, selected cases have been treated by leaving all or part of
the placenta in situ, with or without the addition of systemic methotrexate. The adherent placenta is
either resorbed or passed later. Some patients have had subsequent normal pregnancies, including vaginal
delivery, following such conservative management. No randomized clinical trials, cohort or case-control
studies address the efficacy of this approach, however.
The main pathologic finding in hysterectomy specimens performed for placenta accreta is the presence
of chorionic villi adjacent to (accreta) or invasive into (increta) or through (percreta) myometrium. No
decidual layer is present between chorionic villi and myometrium although there may be a layer of
intervening connective tissue which contains scattered decidual cells. Intermediate trophoblast may be
seen between villi and myometrium and may be difficult to differentiate from decidua on H&E
sections. Trophoblast is cytokeratin positive and individual cells are enveloped by reticulin in
contrast to decidual cells which are cytokeratin negative without surrounding reticulin. There is no
evidence of hyperplasia of trophoblast in this disorder. Villous morphology is normal for gestational
age. Myometrial fibers adjacent to villi may be degenerative or hyalinized. There may be increased
amounts of connective tissue and acute and chronic inflammatory cells between myocytes.
It is often difficult for the pathologist to make a diagnosis of placenta accreta. The placenta and
even the uterus, when it is received, are often quite disrupted due to multiple attempts at manual
removal. Thorough sampling of the basal plate of the placenta and the likely implantation site in the
uterus, concentrating on the area of prior C-section scar, may reveal villi adjacent to myometrium
without intervening decidua. The inability to document this histologically does not, of course, exclude
a diagnosis of accreta. Conversely, at least one study has shown that careful sampling of the basal
plate may reveal mild cases of placenta accreta that were not clinically suspected, which may have
implications for future gestations. En face sections of the basal plate or sections from the junction
between intact and macroscopically disrupted areas have the highest yield for myometrial fibers.
The pathogenesis of this condition appears to relate to an abnormality of the endometrium such that
insufficient decidual tissue forms. Implantation in non-fundic or scarred areas of the uterus or in
areas of thinned or inflamed endometrium, such as over a submucous fibroid, represent such areas of
abnormal endometrium. In a recent study, Sherer et al observed increased placental basal plate
myometrial fibers in cases with preterm delivery and decreased placental weight often adjacent to
abnormal basal plate vessels. They have suggested that early hypoxia may increase trophoblast invasion
resulting in myometrium adherent to the basal plate. How this finding relates to the similar morphologic
findings in placenta accreta is still unclear and has been questioned by others.
Abnormalities of Placental Shape
The placenta is usually a round to oval-shaped organ in which the umbilical cord inserts slightly off
to one side of center. One of the most common abnormalities of shape is the presence of an accessory or
succenturiate lobe. These are usually single but may be multiple and are found in about 3 to 5% of
placentas. The accessory lobe may be separated from the main placenta by a thin amount of villous tissue
or by fetal membranes devoid of underlying villi. Usually the umbilical cord inserts into the main body
of the placenta and the succenturiate lobe is supplied by branches of the umbilical vessels that run,
unprotected, in the connecting membranes. Succenturiate lobes are usually without clinical consequence
but may be retained in utero where they can cause postpartum bleeding and infection or present as
placenta previa. Uncommonly, the unprotected vessels between the main placenta and the succenturiate
lobe are torn, resulting in fetal hemorrhage, or undergo thrombosis. The pathogenesis of this anomaly is
unknown but theories include superficial implantation of the fertilized ovum which then adheres to both
uterine walls, implantation into the lateral or apical sulcus, and failure of villous atrophy of a focal
area of chorion laeve.
In the bilobate placenta (also called bipartite or placenta duplex) there are two lobes of
approximately equal size separated by fetal membranes or a thin bridge of chorionic tissue. The
umbilical cord usually inserts between the two lobes. The incidence is as high as 4.2% in one study
although in most studies it is much less common. Fujukura et al noted a significant association between
bilobate placenta and multiparity, advanced maternal age, and a history of infertility. Women with
bilobate placentas had significantly more first trimester bleeding, placenta previa and required manual
placental extraction twice as often as women without bilobate placentas. There is no increased fetal
morbidity or mortality with this condition.
Placenta membranacea, also known as placenta diffuse, is a common form of placentation in animals but
is very rare in humans where it is seen in about 1 in 20,000 to 1 in 40,000 deliveries. In this
condition the differentiation into chorion frondosum and chorion laeve, which usually occurs between the
8th and 10th week, does not take place and all and most of the membranes are covered by chorionic villi.
There may be an area of increased thickness of chorionic villi, resembling the placental disc, into which
the umbilical cord inserts. Symptoms include recurrent bleeding in the late first and early second
trimester probably due to villous tissue near the os. Most cases are associated with preterm delivery
and fetal mortality, but Ahmed and Gilbert-Barnes have recently reported cases with no associated
bleeding and one with a normal term delivery.
Circummarginate and Circumvallate Placenta (Extrachorial Placenta)
In extrachorial placentation the junction between the chorionic membranes and the chorionic plate of
the placenta occurs not at the margin of the placenta, as is the usual situation, but at some distance
from it so that there is placenta uncovered by membranes extending beyond the chorionic plate (hence
extrachorial). In circummarginate placenta this junction forms a thin rim of fibrin and degenerate
material where the fetal vessels of the chorionic plate terminate. In circumvallate placenta a thicker
rim with a fold toward the center of the placenta is seen, histologic sections of which show avascular
chorionic membrane, decidual tissue, villi, often with hyalinization, and sometimes fibrin and blood.
Circummarginate and circumvallate placentation may be partial or complete. Some placentas show a
combination of circummarginate and circumvallate features.
Incidence figures vary widely probably depending on whether partial or complete cases are included.
In one study of 3000 consecutive placentas 13.7% were partial circummarginate, 3.8% were total
circummarginate, 4.5% were partial circumvallate and 2.4% were total circumvallate. Most studies show an
increased incidence of all types of extrachorial placentation with increased parity.
The clinical significance of extrachorial placentation is controversial. Most studies have found no
clinical significance to circummarginate placentation. Total circumvallate placentation has been
associated with recurrent antepartum bleeding, preterm labor and delivery, fetal hypoxia, post partum
hemorrhage and low birth weight infants in some studies. It is thought by some that repeated episodes of
bleeding into the decidua at the margin of the disc (chronic abruption) is responsible for
circumvallation and that this chronic bleeding is associated with the adverse outcomes listed above.
Diffuse chorioamniotic hemosiderosis is highly correlated with circumvallation (see membrane section).
Circumvallation may recur in successive pregnancies.
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