ULTRASOUND CLINICS

Prenatal diagnosis of placenta accreta

By Natan Haratz-Rubinstein, MD, Tracy Shevell, MD, and Fergal D. Malone, MD

A rare but potentially catastrophic obstetrical complication, placenta accreta is on the increase, given the current trend toward elective repeat C/S. Three experts detail the best approach to management, which begins with antenatal ultrasound diagnosis.

 

Key points

Gray-scale and Doppler U/S are still the main diagnostic tools in the prenatal diagnosis of placenta accreta. Whenever placenta previa or an anterior placenta is noted in patients with previous uterine surgery, the sonographic evaluation of the placental-myometrial interface warrants special attention. Magnetic resonance T-2 and Short Tau Inversion Recovery images are particularly helpful in diagnosing placenta accreta since they can demonstrate the presence of placental tissue with high signal intensity, making it easier to distinguish the placenta from the myometrium.

Placenta accreta is a rare but potentially life-threatening complication of pregnancy that is today an increasingly frequent cause of maternal morbidity and mortality. The term refers to any placental implantation that results in its abnormal adherence to the uterine wall. Pathologically, placenta accreta occurs when the decidua basalis is partially or totally absent in conjunction with an imperfect development of the Nitabuch membrane, a fibrinoid layer that separates the decidua basalis from the placental villi.

Three degrees of severity exist, depending on invasiveness. Placenta accreta, or placenta accreta vera, the most superficial type, occurs when chorionic villi become attached to the myometrium. Placenta increta results when placental villi invade the myometrium. Placenta percreta represents the greatest degree of severity. Defined as the penetration of the trophoblast through the myometrium and into or through the peritoneum, it sometimes extends to adjacent structures such as the bladder, often with catastrophic consequences.

Estimates of the incidence of placenta accreta vary widely, ranging from 1 in 540 to 1 in 93,000 deliveries.¹ Complications include postpartum hemorrhage and its resultant coagulopathy, preterm delivery, postpartum curettage and infection, cesarean hysterectomy, and death. One of the most common risk factors for developing this condition is placenta previa, which is clinically evident in approximately 0.5% of pregnancies at term.² Because of the fact that many of these cases become evident only at the first attempt to separate the placenta at delivery, it is essential to attempt to identify antenatally both placenta accreta and its attendant risk factors, the most common of which is concurrent placenta previa.

Diagnosing concurrent placenta previa

Placenta previa is a life-threatening complication of pregnancy, clinically evident in 0.3% to 0.5% of pregnancies at term.² Complications of placenta previa include abnormal placental adherence or accreta, along with bleeding and the aforementioned operative complications. Prenatal diagnosis of this pathology relies on the capacity to visualize the internal cervical os and its relationship with the lowermost edge of the placenta. Thus, antenatally, placenta previa can be categorized in the following manner²:

(1) Low-lying placenta: the lowermost edge of the placenta does not reach the internal cervical os but is within 2 cm of it.

(2) Marginal placenta previa: the lowermost edge of the placenta is at the margin of the internal cervical os.

(3) Total placenta previa: the placenta completely covers the internal cervical os.

Placenta previa is less frequently diagnosed as gestational age advances due to the so-called "placental migration" phenomenon. A diagnosis of placenta previa is unlikely to change after 32 weeks' gestation, however,³ especially in cases of central placenta previa (where the placenta is equidistant between the anterior and posterior uterine walls).^4-6 In a retrospective study of 276 patients with mid-trimester abnormal placentation, Newton and colleagues found that the incidence of placenta previa fell with advancing gestational age from 76% at 17 weeks' gestation to 3% at term.⁷ Nonetheless, patients in this study with sonographically "resolved" placenta previa were still at significant risk for third-trimester bleeding, abruption, cesarean section, and increased blood loss with prolonged hospitalization.

Other risk factors

The major risk factor for abnormal placental implantation is the combination of an anterior placenta previa with a uterine scar that usually resulted from a previous C/S delivery. Placenta previa itself raises the risk for accreta due to implantation over a highly vascular, poorly contractile lower uterine segment; an existing scar in this same area, as well, obviously compounds the risk. Clark and colleagues demonstrated the effect of previous C/S deliveries on the incidence of placenta accreta (Table 1).⁸ They showed that the risk of placenta previa increases proportionately with the number of previous C/S deliveries (0.26% in an unscarred uterus, and up to 10% in women with four or more previous C/S). The researchers further showed that the association of placenta previa and a previous C/S delivery greatly increases the chances of developing placenta accreta. Patients with no prior uterine scarring had only a 5% incidence of accreta, compared with "veterans" of four C/S deliveries, who had an incidence of up to 67% (Table 1).

Other risk factors for placenta accreta include Asherman's syndrome, submucous leiomyomata, advanced maternal age, multiparity, and prior uterine scarring brought on by previous myomectomy or reconstructive uterine surgery (Table 2).^1,9,11 All of these factors distort the uterine cavity and endometrial environment at the time of implantation. Although it's been hypothesized that advanced maternal age in some way alters the nature of the endometrium, thereby increasing the risk for abnormal placentation, the exact etiology of such a change has yet to be elucidated.

 

TABLE 2 Risk factors for the development of placenta accreta

Placenta previa Previous cesarean delivery Previous myomectomy or reconstructive uterine surgery Asherman's syndrome Multiparity Advanced maternal age Submucous leiomyomata

Prenatal diagnosis

Because of the potential life-threatening complications of placenta accreta, it is imperative that obstetricians recognize its increasing frequency, risk factors, and the diagnostic modalities that are available. Due to the close relationship between the two conditions, we will first describe the prenatal diagnosis of placenta previa pathology using different imaging modalities, before discussing that for placenta accreta.⁸

Transabdominal sonography. Using a transabdominal probe, first locate the placenta, most of which is possible to evaluate in cases of anterior or fundal placentas. Use gray-scale, Doppler, or power amplitude ultrasound (power Doppler) modalities to scan the placental surface in both the sagittal and coronal planes, paying special attention to imaging the placental-myometrial interface. In cases where the placenta is posterior, the same technique applies. Keep in mind that shadowing from the fetus can be a problem, especially in the third trimester.

Although the prenatal diagnosis of placenta previa was initially accomplished using primarily transabdominal sonography, this technique has several practical disadvantages that now limit its use. These are:

(a) the need for a sonographic "acoustic window" with regards to the bladder. A bladder that is too full may distort the lower uterine segment by displacing it posteriorly; thus a low-lying placenta may erroneously appear to be covering the internal os. Conversely, an empty bladder may result in poor imaging quality of the same area, precluding an accurate diagnosis;

(b) shadowing from the symphysis pubis or the fetus;

(c) suboptimal resolution when imaging patients who are obese; and

(d) the presence of myometrial contractions that can distort the internal contour of the uterus, resulting in false-positive diagnoses.^12,13

Transperineal sonography. This modality has also been used to visualize the internal os and its relationship with placental location. Using this approach, Hertzberg and colleagues correctly excluded the presence of placenta previa in 154 women.¹⁴ Of 10 patients in whom placenta previa was diagnosed sonographically, nine had the diagnosis confirmed at delivery. This technique has fallen out of favor, however, given the higher sensitivity and specificity of transvaginal sonography.

Transvaginal sonography. This simple, widely available technique is now the preferred route for evaluating a patient suspected of having placenta previa. TVS is highly accurate with a sensitivity of 87.5%, a specificity of 98.8%, and positive and negative predictive values of 93.3% and 97.6% respectively.^15,16

Using a transvaginal probe, the cervix is evaluated in the sagittal plane. Conventionally, the image is oriented with the patient's bladder on the left side of the screen. A small amount of urine in the bladder is desirable to help delineate the anterior cervical lip. Once the whole cervix is seen, the relationship between the internal cervical os and the placenta can be ascertained.

Although it may appear dangerous to introduce an U/S probe into the vagina of patients with placenta previa, this technique has been shown to be safe.¹⁷ The probe is placed under direct visualization and does not need to touch the cervix to obtain an adequate image. In fact, since the focal length of the probe is 2 to 3 cm, placing the probe too close to the cervix will blur the image. In addition, since the longitudinal axis of the vagina and the cervix are different, it would be highly improbable to reach the internal cervical os using this route.¹⁷ Once the clinician obtains an image of the internal cervical os, he or she can easily ascertain its relationship to the lowermost placental edge.

Sonographic signs of placenta accreta

Placenta accreta can be diagnosed using gray-scale sonography, color Doppler sonography, and power Doppler.

Gray-scale sonography. It's extremely important to diagnose placenta accreta prenatally, so that there's enough time to develop a comprehensive management plan. Due to its wide availability, gray-scale ultrasound has been the cornerstone in diagnosing placenta accreta. Sonographic findings associated with placenta accreta include (Table 3):

(1) a loss of the normally visible retroplacental hypoechoic zone, which most likely corresponds to the decidua basalis, myometrium, and dilated venous channels (Figures 1 and 2),^10,18

(2) progressive thinning of the retroplacental hypoechoic zone on serial exams,¹⁹

(3) the presence of multiple placental lakes that may represent dilated vessels extending from the placenta through the myometrium—the so-called "Swiss cheese" appearance of the placenta (Figure 3),¹⁹

(4) thinning or focal disruption of the uterine serosa-bladder wall complex (percreta), and

(5) focal mass-like elevation of tissue with the same echogenicity as the placenta beyond the uterine serosa (percreta).²⁰

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Click here to view full-size graphic

Click here to view full-size graphic

Color Doppler sonography. Doppler ultrasound has also been suggested as an aid in diagnosing placenta accreta because it highlights areas of increased turbulent flow that may extend from the placenta into the surrounding uterine wall and cervix. The sensitivity and specificity of color Doppler imaging for diagnosing placenta previa accreta, especially anterior placenta accreta, have been high, because it can detect with a high level of confidence abnormal uteroplacental hypervascularity caused by the angiogenesis of placental invasion.

Lerner and colleagues reported a sensitivity of 100% and a specificity of 94% for the prenatal detection of placenta accreta using color Doppler.²¹ Also, this technique may allow turbulent flow to be visualized in cases of placenta percreta where placental vessels extend beyond the uterine serosa and reach other pelvic organs, such as the bladder. Chou and colleagues have characterized the following color Doppler criteria as suggestive of placenta previa accreta (Table 4)²²:

(1) A diffuse lacunar flow pattern from dilated vascular channels scattered throughout the whole placenta and the surrounding myometrial or cervical tissues (Figure 4). High velocity pulsatile venous-type flow can be found in the sonolucent vascular spaces. A finding of this nature has been associated with a higher level of maternal morbidity and mortality.

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(2) A focal lacunar flow pattern showing irregular sonolucent vascular lakes with turbulent lacunar flow distributed regionally or focally within the intraparenchymal placental area.

(3) Interface hypervascularity with abnormal blood vessels linking the placenta to the bladder.

(4) Markedly dilated peripheral subplacental vascular channels with pulsatile venous-type flow over the uterine cervix.

(5) Absence of subplacental vascular signals in the areas lacking a peripheral hypoechoic zone.

 

TABLE 4 Color Doppler signs suggestive of placenta accreta

Dilated vascular channels with diffuse lacunar flow Irregular vascular lakes with focal lacunar flow Hypervascularity linking placenta to bladder Dilated vascular channels with pulsatile venous flow over cervix Poor vascularity at sites of loss of hypoechoic zone. Source: Modified from Chou MM, et al.22

Using these criteria, these investigators reported a sensitivity of 82.4% and a specificity of 96.8% for the antenatal diagnosis of placenta previa accreta.²² The positive and negative predictive values were 87.5% and 95.3%, respectively. Therefore, color Doppler may offer an advantage over gray-scale U/S in that the specificity may be higher and the depth of invasion may be better assessed. In any case, color Doppler is generally used as an adjunct technique to evaluate suspicious findings seen with gray-scale sonography.

Power Doppler. Power amplitude ultrasonic angiography (power Doppler) has also been used to better delineate the abnormal placental vasculature in cases of placenta accreta (Figure 5).²³ Unlike conventional color Doppler imaging, this technique is less dependent on the orientation of the blood vessel. This may result in faster, easier, and more confident acquisition of good Doppler signals. Although flash artifact is a problem with power amplitude ultrasonic angiography in fetal imaging, a clinician can overcome this obstacle by using the breath-holding technique and by keeping the color box as small as possible in the targeted area. Although this is a promising technique, prospective studies comparing its effectiveness with gray-scale ultrasonography and conventional color Doppler have yet to be published.

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Magnetic resonance imaging for placenta accreta

Magnetic resonance imaging is also useful as an adjunct in the diagnosis of placenta accreta.²⁴ T-2 and Short Tau Inversion Recovery images are particularly useful since they can demonstrate the presence of placental tissue with high signal intensity, making it easier to distinguish the placenta from the myometrium.²⁵ Levine and colleagues studied 19 patients at risk for developing placenta accreta with both U/S and MRI.²⁶ Five cases of lower-uterine segment placenta accreta were diagnosed with a high level of confidence using vaginal and power Doppler U/S. In one patient with a posterior placenta who had previously undergone myomectomy, MRI provided the diagnosis of placenta accreta, which was not well depicted by U/S. The researchers concluded that, in these patients, MRI was particularly helpful in cases of posterior placenta where ultrasonographic evaluation is more difficult.

Other investigators have used MRI to assess bladder invasion in cases of placenta percreta.^25,27 As with power amplitude ultrasonic angiography, prospective studies comparing MRI with gray-scale sonography and conventional color Doppler for the diagnosis of placenta accreta are still lacking. This technique appears safe for the fetus, however, and is a promising advance in prenatal diagnosis. According to the most recent Committee Opinion of the American College of Obstetricians and Gynecologists, MRI is not recommended in the first trimester, but neither has it been associated with any known adverse fetal effects.²⁸ As with the other imaging techniques we've discussed, the role of MRI currently is to complement, rather than replace, information obtained via standard sonographic imaging (Figure 6).

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Conclusions

Placenta accreta is a rare but potentially catastrophic obstetrical complication. We are likely to see a continued increase in its prevalence given the current trend toward elective repeat C/S delivery. Antenatal diagnosis is an invaluable aid in perinatal management, as it allows the clinician to anticipate and recognize complications that otherwise might not be expected. A multidisciplinary team can be assembled during the antepartum period to establish the most appropriate management plan, which often entails invasive preoperative preparation and the availability in the operating room of many different subspecialists.

Gray-scale and Doppler ultrasonography remain the main diagnostic tools in the prenatal diagnosis of placenta accreta. Whenever placenta previa or an anterior placenta is noted in patients with prior uterine surgery, place special attention on the sonographic evaluation of the placental-myometrial interface.

MRI appears to be particularly helpful in cases of posterior placenta and in assessing possible bladder involvement. But it's the combination of different imaging modalities that may prove most effective in diagnosing a condition that requires thorough preoperative preparation to maximize outcome for both the mother and fetus.

REFERENCES

1. Fox H. Placenta Accreta, 1945-1969. Obstet Gynecol Surv. 1972;27:475-490.

2. Cunningham FG, MacDonald PC, Gant NF, et al, eds. Williams Obstetrics. 20th ed. Stamford, Conn: Appleton & Lange; 1997:755-760.

3. Farine D, Peisner DB, Timor-Tritsch IE. Placenta previa—is the traditional diagnostic approach satisfactory? J Clin Ultrasound. 1990; 18:328-330.

4. Varma TR. The implication of a low implantation of the placenta detected by ultrasonography in early pregnancy. Acta Obstet Gynecol Scand. 1981;60:265-268.

5. Kurjak A, Barsic B. Changes of placental site diagnosed by repeated ultrasonic examination. Acta Obstet Gynecol Scand. 1977;56:161-165.

6. Gallagher P, Fagan CJ, Bedi DG, et al. Potential placenta previa: definition, frequency, and significance. Am J Roentgenol. 1987;149:1013-1015.

7. Newton ER, Barss V, Cetrulo CL. The epidemiology and clinical history of asymptomatic midtrimester placenta previa. Am J Obstet Gynecol. 1984;148:743-748.

8. Clark SL, Koonings PP, Phelan JP. Placenta previa/accreta and prior cesarean section. Obstet Gynecol. 1985;66:89-92.

9. Tabsh KM, Brinkman CR 3rd, King W. Ultrasound diagnosis of placenta increta. J Clin Ultrasound. 1982;10;288-290.

10. Breen JL, Neubecker R, Gregori CA, et al. Placenta accreta, increta, and percreta. A survey of 40 cases. Obstet Gynecol. 1977;49:43-47.

11. Read JA, Cotton DB, Miller FC. Placenta accreta: changing clinical aspects and outcome. Obstet Gynecol. 1980;56:31-34.

12. Townsend RR, Laing FC, Nyberg DA, et al. Technical factors responsible for "placental migration": sonographic assessment. Radiology. 1986;160:105-108.

13. Artis AA 3rd, Bowie JD, Rosenberg ER, et al. The fallacy of placental migration: effect of sonographic techniques. AJR Am J Roentgenol. 1985;144:79-81.

14. Hertzberg BS, Bowie JD, Carroll BA, et al. Diagnosis of placenta previa during the third trimester: role of transperineal sonography. AJR Am J Roentgenol. 1992;159:83-87.

15. Farine D, Fox HE, Jakobson S, et al. Vaginal ultrasound for diagnosis of placenta previa. Am J Obstet Gynecol. 1988;159:566-569.

16. Leerentveld RA, Gilberts EC, Arnold MJ, et al. Accuracy and safety of transvaginal sonographic placental localization. Obstet Gynecol. 1990;76:759-762.

17. Timor-Tritsch IE, Yunis RA. Confirming the safety of transvaginal sonography in patients suspected of placenta previa. Obstet Gynecol. 1993;81:742-744.

18. Pasto ME, Kurtz AB, Rifkin MD, et al. Ultrasonographic findings in placenta increta. J Ultrasound Med. 1983;2:155-159.

19. Hoffman-Tretin JC, Koenigsberg M, Rabin A, et al. Placenta accreta. Additional sonographic observations. J Ultrasound Med. 1992;11:29-34.

20. Finberg HJ, Williams JW. Placenta accreta: prospective sonographic diagnosis in patients with placenta previa and prior cesarean section. J Ultrasound Med. 1992;11:333-343.

21. Lerner JP, Deane S, Timor-Tritsch IE. Characterization of placenta accreta using transvaginal sonography and color Doppler imaging. Ultrasound Obstet Gynecol. 1995;5:198-201.

22. Chou MM, Ho ES, Lee YH. Prenatal diagnosis of placenta previa accreta by transabdominal color Doppler ultrasound. Ultrasound Obstet Gynecol. 2000;15:28-35.

23. Chou MM, Ho ES. Prenatal diagnosis of placenta previa accreta with power amplitude ultrasonic angiography. Am J Obstet Gynecol. 1997;177:1523-1525.

24. Silver LE, Hobel CJ, Lagasse L, et al. Placenta previa with bladder involvement: new considerations and review of the literature. Ultrasound Obstet Gynecol. 1997;9:131-138.

25. Thorp JM Jr, Councell RB, Sandridge DA, et al. Antepartum diagnosis of placenta previa percreta by magnetic resonance imaging. Obstet Gynecol. 1992;80:506-508.

26. Levine D, Hulka CA, Ludmir J, et al. Placenta accreta: evaluation with color Doppler US, power Doppler US, and MR imaging. Radiology. 1997;205:773-776.

27. Bakri YN, Rifai A, Legarth J. Placenta previa-percreta: magnetic resonance imaging findings and methotrexate therapy after hysterectomy. Am J Obstet Gynecol. 1993;169:213-214.

28. American College of Obstetricians and Gynecologists, Committee on Obstetric Practice. Guidelines for Diagnostic Imaging During Pregnancy. ACOG Committee Opinion. No. 158, September 1995.

Dr. Haratz-Rubenstein is Assistant Professor of Ob/Gyn; Dr. Shevell is a Maternal-Fetal Medicine Fellow; and Dr. Malone is Assistant Professor, Division of Maternal-Fetal Medicine, Columbia University College of Physicians and Surgeons, New York Presbyterian Hospital, New York, N.Y.

Department editors are Mary E. D'Alton, MD, Virgil G. Damon Professor of Obstetrics and Gynecology and Director of Obstetrics and Maternal-Fetal Medicine, Columbia University College of Physicians and Surgeons, New York Presbyterian Hospital, New York, N.Y., and Richard L. Berkowitz, MD, Professor and Chairman, Department of Obstetrics, Gynecology, and Reproductive Science, Mt. Sinai Medical Center, New York, N.Y.