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Keywords:

  • placenta accreta;
  • placenta previa;
  • ultrasound

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

Objective

To examine the diagnostic precision of ultrasound examination for placenta accreta in women with placenta previa and to compare the morbidity associated with accreta to that of previa alone.

Methods

This was a retrospective cohort study of all women with previa with/without accreta examined at the University of California, San Francisco (UCSF) between 2002 and 2008. The sensitivity, specificity, negative predictive value (NPV) and positive predictive value (PPV) of ultrasound examination for the diagnosis of accreta were calculated and compared with results from similar studies in the literature. Univariable analysis was used to compare clinical outcomes.

Results

The PPV of an ultrasound diagnosis of accreta was 68% and NPV was 98%. Ultrasound had a sensitivity of 89.5%. Compared with previa alone, accreta had an odds ratio (OR) of 89.6 (95% CI, 19.44–412.95) for estimated blood loss > 2 L, an OR of 29.6 (95% CI, 8.20–107.00) for transfusion and an OR of 8.52 (95% CI, 2.58–28.11) for length of hospital stay > 4 days.

Conclusion

Placenta accreta is associated with greater morbidity than is placenta previa alone. Ultrasound examination is a good diagnostic test for accreta in women with placenta previa. This is consistent with most other studies in the literature. Copyright © 2011 ISUOG. Published by John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

Placenta accreta is a pathological condition in which the placental trophoblast invades the endometrium beyond the Nitabuch's layer due to a defect in the decidua basalis1. In more severe cases, the trophoblast invades the myometrium (placenta increta) or the serosa and beyond (placenta percreta). The major morbidity associated with such abnormal placentation primarily arises from the significant blood loss that occurs at the time of delivery, including increased maternal hospital stay, estimated blood loss and need for a blood transfusion2. In addition, pregnancies complicated by placenta accreta are thought to be associated with increased incidences of cystotomy, ureteral injury, pulmonary embolism, need for ventilator use, reoperation, and intensive care unit (ICU) admission3.

Risk factors for placenta accreta include prior Cesarean delivery, uterine instrumentation and intrauterine scarring, all of which may be associated with damage to or absence of the decidua basalis, as well as placenta previa, smoking, maternal age over 35, grand multiparity and recurrent miscarriage1, 4, 5. Given the continual increase in Cesarean deliveries both in the USA and worldwide, the relationship between prior uterine surgery and the risk of placenta previa and accreta is increasingly important. It has been noted that one prior Cesarean delivery doubles the risk of placenta previa in a subsequent pregnancy such that the incidence increases from 0.38 to 0.63%6; this effect is further compounded such that women with a placenta previa in the setting of prior uterine surgery are particularly at risk for accreta. For example, in a patient with no prior Cesarean and a placenta previa in the current pregnancy, the risk of placenta accreta is 3.3%, this increases to 11% with one prior Cesarean and a current previa and goes up even further to 40% with two prior Cesarean deliveries and a placenta previa in the current pregnancy3.

Given the significant morbidity associated with this diagnosis, the ability to accurately diagnose placenta accreta is essential as it allows both the patient and the provider to be prepared for the potential complications of delivery. Ultrasound can be used for the prenatal assessment of possible placenta accreta; however, evidence regarding the sensitivity and specificity has been mixed. While some have reported ultrasound to have a sensitivity of 77–93% and a specificity of 71–98%7–10, others have reported a much lower sensitivity of 33% for ultrasound11. Similarly, although several groups have attempted to look at the morbidity associated with placenta accreta, the overall number of cases in the literature is low.

Given this background, we sought to examine the performance of ultrasound examination as a diagnostic tool for placenta accreta in women with placenta previa, to compare our findings with those of others in the literature and to compare the incidence of complications between cases of placenta accreta and of placenta previa alone. Patients with placenta previa alone were chosen as a comparison group because they are the population at greatest risk for placenta accreta, thereby underscoring the importance of making an accurate diagnosis in these patients.

Patients and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

This was a retrospective cohort study of women with placenta previa identified at the University of California, San Francisco (UCSF) between 2002 and 2008. Patients were identified through an existing clinical database and their charts were abstracted to determine whether they had an ultrasound examination in the third trimester and whether the prenatal diagnosis of placenta accreta was confirmed at the time of surgery and then again on pathology if a pathological specimen was available. Ultrasound studies at our institution are routinely performed using both grayscale and color Doppler sonography. If suspicion is raised and the suspected focus of accreta is in the lower uterine segment, transvaginal ultrasound is also performed. Findings suggestive of placenta accreta included: loss of the bladder wall–uterine border, adjacent placental sonolucent spaces or placental lakes, and increased vascularity next to the bladder wall, as documented by color Doppler12–14 (Figure 1). Patients were excluded from the study if they did not have any imaging performed at UCSF antenatally or if they did not deliver at UCSF.

thumbnail image

Figure 1. Ultrasound findings suggestive of placenta accreta include: loss of bladder wall–uterine border (a); placental lacunae (b); increased vascularity on color Doppler next to the bladder wall (c). The calipers in (a) are measuring the cervix.

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The electronic medical record was also used to determine the estimated blood loss (EBL), presence or absence of Cesarean hysterectomy, ICU admission, length of stay, the need for transfusion of packed red blood cells (PRBC), coagulation factors and platelets, as well as the amount of blood products transfused in each case. Cases using Cell Saver® were identified and the amount of Cell Saver blood transfused recorded. Patients with the presence of accreta were also compared with those with complete placenta previa in the third trimester with regard to EBL ≥ 2 L, need for a PRBC transfusion, and length of stay > 4 days.

Statistical analysis was performed using STATA v9.0 (StataCorp, College Station, TX, USA) statistical software. Binary diagnostic test tables were used to calculate sensitivity, specificity, negative predictive value (NPV), positive predictive value (PPV) and false-positive rate. Statistical significance was considered if P < 0.05. Univariable analysis was used to calculate the medians, odds ratios and 95% confidence intervals (CI).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

In total, there were 108 women with placenta previa during the study period. All had at least one ultrasound examination. Of the total number of women, 25 were diagnosed as having placenta accreta (all on the anterior uterine wall) on ultrasound and of these, 17 cases were confirmed at the time of surgery and later on pathology. In 83 women, ultrasound examination was negative for placenta accreta, but of these, two cases of accreta were found at surgery and on pathology. In our cohort, all but one patient had a hysterectomy. The sensitivity and specificity of ultrasound in diagnosing placenta accreta were 89.5% (17/19) and 91% (81/89), respectively, with a false-positive rate of 9%. Similarly, the PPV for ultrasound was 68% (17/25) and the NPV was 97.6% (81/83) (Table 1).

Table 1. Test characteristics of ultrasound for the diagnosis of placenta accreta in our study and others from the literature7, 8, 10, 11
StudySens. (%)Spec. (%)PPV (%)NPV (%)
  1. NPV, negative predictive value; PPV, positive predictive value; Sens., sensitivity; Spec., specificity.

This study (n = 108)89.591.068.097.6
Chou et al.7 (n = 80)82.496.887.595.3
Warshak et al.8 (n = 453)77.096.065.098.0
Dwyer et al.10 (n = 32)93.071.074.092.0
Lam et al.11 (n = 13)33.0100.0100.011.1

In terms of maternal morbidity, 93.8% of women with a placenta accreta vs. none with placenta previa alone underwent a Cesarean hysterectomy (P < 0.001). In addition, 38.8% of women with placenta accreta vs. none of the women with placenta previa alone experienced an ICU admission (P < 0.001), and 12.5% of women with placenta accreta vs. none of the women with placenta previa alone (P < 0.001) had an injury to a surrounding organ. Seventy-nine percent of women in the accreta group (compared with 11% in the previa group) needed a PRBC transfusion (P < 0.001). Medians were calculated for the amount of PRBCs, fresh frozen plasma (FFP) and platelets because the distribution of the values was not normal. The median EBL was 3000 mL in the accreta group with a range of 1350–11 000 mL vs. a median EBL of 1000 mL in the placenta previa group with a range of 700–3000 mL (P < 0.001). The median PRBC amount was 3 units with a range of 0–17 units for the accreta group and 0 units with a range of 0–5 units for the previa group (P < 0.001). Although the median FFP and platelet amounts for both groups was 0, the ranges were quite statistically significantly different, 0–14 units of FFP for accretas vs. 0 units of FFP for previas (P < 0.001) and 0–6 units of platelets for accretas vs. 0 units of platelets for previas (P = 0.005).

Accreta and previa without accreta cases were also compared in terms of median EBL > 2 L, the need for PRBCs and length of stay > 4 days using odds ratios (OR) and confidence intervals (CI). The OR for median EBL > 2 L was 89.6 (95% CI, 19.44–412.95), for need for PRBC transfusion was 29.63 (95% CI, 8.20–107.00) and for length of stay > 4 days was 8.52 (95% CI, 2.58–28.11).

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

Although there still appears to be a difference of opinion in the literature regarding the accuracy of ultrasound for the diagnosis of placenta accreta, our sensitivity of 89.5% and specificity of 91% support several previous reports with similar findings. With the exception of the Lam study, all report a sensitivity of 77–93%, specificity of 71–97%, PPV of 65–88% and NPV of 92–98%, as demonstrated in Table 17, 8, 10, 11.

Although a PPV of 68% is not ideal, taking into account the pretest probability of accreta in patients with prior uterine surgery and a previa, this still provides useful clinical information. In addition, the NPV of 97.6% suggests that the true value of imaging may be to rule out placenta accreta. In this clinical setting, where increased preparation for delivery is the most likely consequence of a positive test result, these test characteristics are useful. Most clinicians would agree that being over-prepared for some cases is acceptable, as long as they can be reasonably certain that if they have a negative ultrasound result, they will not be faced with an undiagnosed accreta at the time of delivery. Therefore, although not perfect, ultrasound appears to be a powerful and fairly inexpensive testing modality in the diagnosis of placenta accreta.

Both placenta previa and placenta accreta involve abnormal placentation, but placenta accreta appears to carry a significantly higher risk to the patient. Indeed, we observed higher incidences of Cesarean hysterectomy, need for PRBC transfusion, intraoperative organ injuries and need for ICU admission. Although there are reports that conservative management with leaving placenta in situ is an option and may decrease blood loss and other perioperative morbidity in select patients15, most providers still choose to go ahead with a hysterectomy and reserve this management option only for patients who have minimal blood loss and strongly desire fertility preservation16. There is now emerging evidence that delayed postpartum evaluation of the remaining in utero placenta and the uterus with color Doppler may assess the amount of involution that has occurred and help determine if there is need for adjuvant therapy17. Regardless of whether surgical or conservative management is undertaken, informing patients of these risks is an important part of counseling regarding a pregnancy potentially complicated by placenta accreta. In addition, this information should alert the provider for preoperative preparation and planning to manage the potential surgical morbidities; these include having the appropriate and sufficient blood products available at the time of delivery, and arranging for a skilled team to be available for the procedure, which includes experienced surgeons, anesthesiologist, nursing staff and appropriate consultants, as well as the ICU.

Although, until recently, placenta accreta was considered a fairly rare event, its annual incidence appears to be on the increase. In 1994, the incidence of placenta accreta for the previous 10 years was reported to be 1 in 2510 cases, whereas a study in 2002 reported an incidence of 1 in 533 cases for the previous 20 years18, 19. This alarming increase appears to be directly related to the rising rates of Cesarean delivery, a major risk factor for placenta previa and accreta. In fact, a model looking at what would happen if the Cesarean rate increased from 29 to 34%, predicted a tremendous increase in various morbidities, including an increase in surgical complications by almost 20 000 cases and an increase in spending of over 1 billion healthcare dollars20.

Although we were able to determine that ultrasound has reasonable test characteristics for the diagnosis of placenta accreta, our study does have limitations. This was a retrospective study and thus neither randomized nor blinded; in addition our sample size was fairly small. Furthermore, our patient population is from a tertiary care center. While we observed an excellent negative predictive value of ultrasound, we recognize that the physicians who read ultrasounds at our institution are experts in the field. Thus, the results may not be generalizable to all providers. It is, therefore, reasonable for community providers to refer their patients to a tertiary center for a consultation. Finally, because the providers were not blinded by history, diagnostic bias could have been introduced into our study.

Although there is evidence that even in cases of non-emergent hysterectomy the morbidity related to blood loss and ureteral injury remains significant21, accurate prenatal diagnosis plays a significant role in decreasing the morbidity associated with placenta accreta. Recent work demonstrated that planned Cesarean hysterectomy in suspected cases of placenta accreta significantly decreased the rate of early morbidity (prolonged maternal ICU admission, large volume of blood transfusion, coagulopathy, ureteral injury or early reoperation) compared with cases of unsuspected accreta, where placental removal was often attempted22. Once the diagnosis is suspected, adequate preparation is crucial, starting with having the appropriate staff, sufficient blood products and Cell Saver readily available. In addition, interventional radiology has come to play a key role in attempting to decrease the morbidity related to placenta accreta and is utilized both pre- and postoperatively. New findings point to the use of a staged embolization hysterectomy procedure for placenta accreta as significantly decreasing maternal morbidity, especially in terms of blood loss23. Placenta accreta is undoubtedly a challenge, but with proper diagnosis and preparation, the goal is to decrease the morbidity of this rapidly increasing obstetric complication.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES

This study was presented as the Frank Lynch Memorial Essay at the 2009 Pacific Coast Obstetrical and Gynecological Society Meeting. A.B.C. is supported by the Robert Wood Johnson Foundation as a Physician Faculty Scholar, Grant # RWJF-61535.

REFERENCES

  1. Top of page
  2. Abstract
  3. Introduction
  4. Patients and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. REFERENCES
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