The Alarming Rise of Adherent Placenta – The Risk of Cesarean Section
Soon after a woman gives birth, the placenta separates from the uterus, is delivered and is commonly referred to as the afterbirth. The complete delivery of the placenta is a critical step in preventing post-partum bleeding. During pregnancy, the uterus develops an enormous network of blood vessels that bathe the under-surface of the placenta, and supply essential nutrients to the growing baby for nine months. Near the end of the pregnancy, the average uterine blood flow is estimated to be between 500cc and 800cc per minute. With the separation of the placenta, the dramatic contraction of the uterus as it reduces in size, cuts off the blood supply to these vessels and prevents excessive bleeding. Life-threatening hemorrhage occurs when the placenta is so adherent to the uterus that it does not spontaneously separate, thus preventing the normal involution of the uterus. The resulting hemorrhage is an obstetrical emergency with over 90% of women requiring a blood transfusion and 40% of women requiring more than ten units of blood. This condition is referred to as either a morbidly adherent placenta (MAP) or abnormally invasive placenta (AIP).
The degree of placental attachment is classified as follows:
- Accreta (81.6%): the placenta is firmly attached to the uterine muscle. (This term is often used interchangeably with MAP).
- Increta (11.8%): the placenta has invaded the muscle of the uterus
- Percreta (6.6%): the placenta completely invades the uterine muscle and occasionally into adjacent pelvic organs such as the bladder or colon.
A morbidly adherent placenta (MAP) can be total, partial or focal:
- Total placenta accreta involves the entire placenta.
- Partial placenta accreta involves only a portion of the placenta that consists of at least two lobes.
- Focal placenta accreta involves only a single placental lobe.
Only recently, has placenta accreta been studied and described by pathologists. Unlike many other placental disorders, which have been reported for centuries, placenta accreta was ﬁrst described in 1937 by Irving and Hertig. The fact that major changes in Cesarean surgical techniques preceded the ﬁrst detailed description of a placenta accreta by a couple of decades is indicative of a direct relationship between prior uterine surgery and abnormal placenta adherence. The rise in the rate of cesarean sections over the past 20 years (now 32%-33% of all deliveries) parallels a dramatic and alarming increase in the incidence of MAP. While the incidence of MAP will vary by the population and prevalence of cesarean section, a recent large US based analysis revealed the following temporal changes in this obstetrical complication:
- 1950’s – 1/19,000 pregnancies
- 1970’s – 1/4027 pregnancies
- 1980’s – 1/2057 pregnancies
- 2005 – 1/533 pregnancies
- 2010 – 1/333 pregnancies
No longer considered a rare event, placenta accreta is rapidly becoming recognized as a major cause of obstetric morbidity and mortality worldwide. Another complication associated with a history of a previous cesarean section is placenta previa. Placenta previa is a condition where the placenta attaches either partially or wholly within the lower uterine segment and is a major cause of antepartum maternal hemorrhage. Placenta previa complicates approximately 0.5% of all pregnancies. However, a history of a single previous cesarean section increases the risk of this complication tenfold to an incidence of 5%. For women with one incidence of previa and no history of cesarean section, the risk of accreta is 3%, and the incidence rises to 11% with a history of one cesarean section. With a history of two and three cesarean sections and placenta previa, the incidence of accreta is an alarming 40% and 61% respectively.
Recent data from a large prospective cohort study reported the increase risk of placenta accreta as it relates to the number of previous caesarean sections, in the absence of placenta previa as follows:
- One previous cesarean section – 2.5x increased risk
- Two previous cesarean sections – 5x increased risk
- Three previous cesarean sections – 7x increased risk
Another large survey from the United Kingdom found a three-fold increase in placenta accreta in twin pregnancies and an astounding 32 fold increase risk of accreta in pregnancies achieved through the use of assisted reproductive technologies (ART). A recent study still In Press in the journal Fertility and Sterility, found that even after controlling for patient age, prior cesarean section, placenta previa, and uterine factor infertility, conception following a frozen embryo transfer appears to be a strong independent risk factor for placenta accreta. The authors postulate that this increase may be the result of a combination of an unresponsive, thin endometrium and low estradiol levels. These findings are consistent with a previous report of over 200,000 embryo transfer cycles from Japan, which also showed a greater than three-fold increased risk of accreta in pregnancies conceived after a frozen embryo transfer cycle. The data linking ART to accreta is however limited to only these few reports, and while it warrants increased vigilance, further study is needed to confirm these findings.
Paralleling the rise in MAP is the incidence of cesarean scar pregnancy (CSP). It is now believed that these pregnancies give rise to placenta accreta. A recent publication described ten patients with CSP, who chose to continue their pregnancy. Nine of the ten patients delivered a live-born infant between 32 and 37 weeks. One patient was forced to terminate the pregnancy at 20 weeks due to uncontrolled vaginal bleeding. All ten patients developed MAP and required a hysterectomy at delivery to control bleeding. Blood loss ranged from 300ml to 6000ml. After an examination of the hysterectomy specimens, the histopathology in all 10 case was placenta percreta. The authors postulate that CSP is the likely precursor to most cases of MAP. A recent review of 47 cases of CSPs, highlights the additional risk of spontaneous uterine rupture. Of thr 47 patients diagnosed with CSP and who elected to continue with the pregnancy, 15 (32%) suffered a silent uterine rupture with internal bleeding and shock. All of these patients underwent a therapeutic hysterectomy.
Placenta accreta is a dreaded obstetric complication. The average blood loss at delivery in women with placenta accreta is 3,000–5,000 ml. In some large series, as many as 90% of patients with placenta accreta required a blood transfusion, and 40% required more than ten units of packed red blood cells. Maternal mortality with placenta accreta has been reported to be as high as 7% to 30%, and the majority of patients require a hysterectomy. There is also a significantly increased risk of preterm birth and fetal mortality with placenta accreta. Unexpected accreta has the highest risk of morbidity and mortality. With the establishment of centers of excellence for accreta and the increase in antenatal diagnosis of this condition with both ultrasound and magnetic resonance imaging (MRI), mortality is quite rare with a significant drop in morbidity. There have been attempts at more conservative therapy with the intent of saving the uterus. These attempts have been directed at either removing the placenta or leaving it in place and treating with methotrexate and expectant therapy. Currently, the American College of Obstetrics and Gynecology does not make any specific recommendations regarding placental removal, while the Royal College of Obstetrics and Gynecology recommends that under no circumstance should any attempt be made to remove the placenta. Conservative attempts appear to have a significant risk of increased morbidity with an increased risk of late complications of bleeding, blood loss, and serious infection leading to sepsis. Critics of this strategy point out that the risk of recurrence of accreta is so high in these patients, that this risk precludes the use of conservative therapy.
The incidence of placenta accreta and placenta previa is rising at an alarming rate, paralleling the increased prevalence of cesarean section. It is prudent then to counsel patients about this risk, especially those women undergoing pregnancy as gestational carriers. Typically, infertility patients undergo a series of early trans-vaginal ultrasounds with highly trained professionals who are alert to the possibility of cesarean scar pregnancy. Once these pregnancies are recognized, the patient can elect to terminate the pregnancy in the first trimester, or if this is not an acceptable option, she may elect to continue with the pregnancy. Early diagnosis and management of accreta can greatly reduce a patient’s risk of serious morbidity and mortality; however, patients must be counseled that it will likely be the last pregnancy they will ever carry.
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