Etiology and Management of Placenta Accreta at 20 Weeks
Jul 2011 – by R. Smith, MD
A 23-year-old G3P2011 woman at approximately 20-weeks gestation undergoes a screening ultrasonography for size greater than dates. The placenta is noted to be anterior, low on the uterine wall and there is poor delineation between the placenta and the uterus on the lower aspect.
a. Differential diagnosis. While factors such as abdominal wall thickness, the presence of a pannus, the amount of bladder filling, fetal position or presentation and the technical skill of the sonographer can all conspire to give a less than optimal view of the interface between the uterus and placenta, the possibility of abnormal placentation must be considered. The degree of invasion of the myometrium by the chorionic villi will define if this is a placenta accreta (adherent to the myometrium), increta (invading the myometrium) or percreta (perforating through the myometrium). Uterine rupture with partial expulsion of the placenta could appear similar during ultrasonography, but would seem improbable in the clinical setting given. Conditions such as a sub-chorionic bleed or placental abruption would have a very different appearance on ultrasonography.
b. Etiology. Placenta accreta and its variants all arise when there is abnormal decidua formation at the time of placental implantation, specifically imperfect development of the fibrinoid (Nitabuch's) layer. This failure is more common when there is an 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). For these reasons, risk factors for abnormal placentation include placenta previa (without previous uterine surgery 5%, with previous surgery 15%-70%), previous cesarean delivery, multigravidity (1 of 500,000 for parity <3, 1 of 2500 for parity >6), older pregnant women, previous uterine curettage, previous uterine sepsis or manual removal of the placenta, leiomyomata, uterine malformation, prior abortion, and endometrial ablation. All (total) or one portion (partial) of the placenta may be involved.
c. Diagnosis. When only a small part of the placental disc is involved, the diagnosis may not be made until there is failure of normal separation of the placenta following delivery. This may be accompanied by abnormally heavy bleeding after delivery of the placenta, which may be life threatening. Ultrasonography has been used to make the diagnosis before labor in unusual cases or those who are thought to be at high risk. Low-lying placentas noted in studies performed below 30 weeks may "migrate," leaving the cervix free at term (up to 90% of cases) and the majority of low lying or complete placenta previa patients do not have an abnormality of implantation, only location. When diagnosed by ultrasonography, placenta accreta is suspected by loss or blurring of the normal placenta-uterine wall boundary, the absence of the subplacental hypo echoic zone or the presence of lacunar blood flow patterns. The final diagnosis is established histologically: Absence of the decidua basalis (replaced by loose connective tissue). The decidua parietalis may be normal or absent. The villi may be separated from the myometrial cells by a layer of fibrin.
d. Management. Most patients go to term with normal fetal development. Any time the diagnosis is considered, preparations for hysterectomy, including anesthesia, instruments, and adequate blood, should be ready before any attempt is made to free the placenta. If the diagnosis is suspected sufficiently far in advance, plans for autologous blood donation and elective cesarean hysterectomy may be made. When placenta accreta is diagnosed at delivery, management depends on the clinical situation at hand. Life-threatening hemorrhage may occur; maternal mortality of 2%-6% has been reported for treatment by hysterectomy and up to 30% for conservative management. Rupture of the uterus or inversion may occur during attempts to remove the placenta. Most patients require hysterectomy. Aggressive fluid and blood support must be provided as necessary. Coagulopathy secondary to blood loss and replacement is common. Oxytocin or other uterotonic agents are used to promote uterine contractions after placental delivery (if accomplished). If the invasion of the myometrium is incomplete and the bladder is spared, conservative management by uterine packing may be possible. Spontaneous rupture of the uterus before labor has been reported.