| Patient Education |
|
Click here for patient education.
|
|
You are in: eMedicine Specialties >
Radiology > OBSTETRICS/GYNECOLOGY
Placenta Previa
Article Last Updated: Aug 26, 2005
AUTHOR AND EDITOR INFORMATION
Section 1 of 11  
Author: Greg Marrinan, MD, Staff Physician, Department of Radiology, Bridgeport Hospital
Greg Marrinan is a member of the following medical societies: American College of Radiology, American Medical Association, American Roentgen Ray Society, North American Society for Cardiac Imaging, and Radiological Society of North America
Coauthor(s):
Marjorie Stein, MD, Clinical Assistant Professor of Radiology, Albert Einstein College of Medicine; Consulting Staff, Department of Radiology, Montefiore Medical Center
Editors: Christopher L Sistrom, MD, Associate Chair for Research, Assistant Professor, Department of Radiology, University of Florida School of Medicine; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Karen L Reuter, MD, FACR, Professor, Department of Radiology, Lahey Clinic Medical Center; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Eugene C Lin, MD, Consulting Staff, Department of Radiology, Virginia Mason Medical Center
Author and Editor Disclosure
Synonyms and related keywords:
abnormal implantation, low implantation, internal cervical os, complete or total placenta previa, incomplete or partial placenta previa, marginal placenta previa, low-lying placenta previa
Background
Placenta previa is a condition in which the placental tissue lies abnormally close to the internal cervical os. The 4 generally recognized subtypes are (1) complete or total, in which the placenta covers 360° of the internal cervical os; (2) incomplete or partial, in which 0°-360° of the internal cervical os is covered by placental tissue; (3) marginal, in which the placental tissue abuts but does not cover the internal cervical os; and (4) low lying, in which the edge of the placenta lies abnormally close to but does not abut the internal cervical os.
The American College of Radiology (ACR) Appropriateness Criteria recommend against the use of the above terms in radiologic reports, because they are "vague and difficult to quantify." Instead, the ACR suggests describing the relationship between the placenta and internal cervical os.
Pathophysiology
Placenta previa typically occurs as a result of abnormally low implantation. Although no specific cause has been identified to date, this condition has been hypothesized to occur as a result of abnormal endometrial vascularization related to atrophy or scarring from prior trauma or inflammation.
As the lower uterine segment thins in late pregnancy, the margins of the abnormally implanted placenta are altered. Various degrees of placental detachment may develop, with ensuing maternal hemorrhage from the intervillous space. During labor, significant fetal hemorrhage also can occur as a result of disrupted villous placental vessels.
Risk factors for placenta previa include prior placenta previa, prior cesarean delivery, increased maternal age, large placentae (eg, multiple gestations or erythroblastosis), and a maternal history of smoking.
Frequency
United States
Placenta previa occurs in 0.3-2.0% of all births. This range in the reported incidence results from differing definitions, methods of diagnosis, and gestational ages at the time of diagnosis. In addition, the frequency varies in different patient populations.
Incidence rates in the United States are primarily derived from retrospective hospital-based studies. In 1991, Iyasu and colleagues reviewed the National Hospital Discharge Survey from 1979-1987 to determine the national incidence of placenta previa. Their results indicate that placenta previa was diagnosed in 0.48% of all births. This statistic agreed with those of previous smaller studies.
International
To the authors' knowledge, no prospective examination of the international frequency of placenta previa has been reported to date. Findings of a small sampling of individual studies from the international literature suggest that the frequency is similar to that in the United States.
Mortality/Morbidity
- Maternal mortality and morbidity: Placenta previa may result in significant maternal hemorrhage during pregnancy. The initial episode of bleeding is almost always self-limited, but subsequent episodes are invariably more serious. The maternal mortality rate is approximately 0.1%. Most often, death is a direct result of uterine hemorrhage or disseminated intravascular coagulopathy. Maternal morbidity is uncommon and usually associated with the complications of cesarean delivery. Conditions include endometritis, urinary tract infection, and postoperative pneumonia. Rarely, amniotic fluid embolization may occur. Hemorrhage that results in shock and/or requires emergency hysterectomy is rare in the absence of a placenta accreta.
- Fetal mortality and morbidity: The incidence of congenital anomalies associated with placenta previa is increased. A number of reports also indicate that placenta previa is associated with intrauterine growth retardation, though this is somewhat controversial. McShane and colleagues found that perinatal anemia and fetal lung immaturity are significant causes of morbidity. Perinatal mortality is directly correlated with the level of fetal lung maturity at the time of delivery. Therefore, the gestational age at which bleeding first occurs is a major factor in the perinatal prognosis.
Race
In the United States, the incidence of placenta previa is reported to be slightly higher in minority populations.
Age
Increased maternal age is a known risk factor for placenta previa.
Anatomy
Approximately 6 days after fertilization, the blastocyst attaches to the decidual cells of the endometrial epithelium. The thin outer layer (ie, trophoblast) rapidly proliferates and differentiates into a cytotrophoblast and a syncytiotrophoblast. Fingerlike processes extend outward from the syncytiotrophoblast, through the decidual layer, and into the endometrial stroma.
Within 2 weeks of fertilization, networks of lacunae form within the syncytiotrophoblast. These spaces are filled with maternal blood derived from ruptured endometrial capillaries. This process is the beginning of uteroplacental circulation, and these lacunar networks eventually form the intervillous spaces of the mature placenta. By the end of the second week, chorionic villi begin to form. These structures form the fetal component of the placenta and project into the intervillous space.
Clinical Details
Placenta previa is an uncommon cause of vaginal bleeding during pregnancy. In fact, less than one half of patients with bleeding have placenta previa. However, of the patients with placenta previa, as many as 70% have painless vaginal bleeding during the second half of pregnancy. A fraction of this group has uterine contractions.
The initial episode of hemorrhage is often unheralded. It can occur without an inciting cause, although pelvic examination, intercourse, or labor may provoke it. The average gestational age at presentation is 32 weeks.
The first event is rarely life threatening, and it tends to cease spontaneously. Usually, the fetus is unharmed by the incident. Hemorrhage recurs, and, in nearly all cases, it is more severe the second time.
Patients are treated expectantly, with volume replacement, transfusions, tocolytics, and emergent cesarean delivery when necessary. Without endangering the life of the mother, all attempts are made to delay delivery until the fetal lungs mature.
Preferred Examination
Historically, placenta previa was diagnosed by means of digital palpation of the placental tissue through the cervical canal. The slightest amount of manipulation, however, can result in a substantial amount of hemorrhage. Physical examination should be performed only with a fetus that has achieved pulmonary maturity and only in a fully staffed operating room. Maternal bleeding may be so severe that immediate delivery is necessary.
Transabdominal sonography is the test of choice to confirm placenta previa. When the internal cervical os cannot be visualized or when the results are inconclusive, transperineal or transvaginal sonography is recommended as an adjunct. The overall accuracy of ultrasonography in the evaluation of placenta previa has been reported as 93-98%. Transperineal studies have a negative predictive value of nearly 100% for this diagnosis. No increased risk of hemorrhage has been associated with transvaginal or transperineal sonography in this clinical setting.
MRI has been used to evaluate placental location in a number of studies. Its accuracy is equivalent to and may even surpass that of sonography. However, the long-term effects of exposure to a static magnetic field and rapidly shifting gradients on fetal development have not been fully evaluated. In addition, the time required to arrange and perform an adequate examination may limit its usefulness, particularly in the setting of acute maternal hemorrhage. At present, the use of MRI in the diagnosis of placenta previa should be limited to a few specific cases, and MRI should be used only after sonography fails to provide adequate information.
Limitations of Techniques
The major limitation of sonography in the diagnosis of placenta previa is related to the gestational age at diagnosis. The reported incidence of placenta previa in the second trimester is nearly 10 times that at delivery. A variety of explanations have been proposed to account for this difference. The most likely theory suggests that, during the third trimester, the lower uterine segment elongates more than the placenta enlarges. Thus, a placenta that appears marginal or low lying at 20 weeks may be normally positioned at term. Results of most investigations of this phenomenon, however, agree that a complete placenta previa in the second trimester rarely reverts to a normal position at term.
Placenta, Abruption
Other Problems to be Considered
Overdistended bladder
Myometrial contraction
Placenta accreta
Vasa previa
Low-lying placenta
Findings
Plain radiography and tomography are of no value in the diagnosis of placenta previa.
Findings
CT plays no role in the diagnosis of placenta previa.
Findings
Usually, the placenta is relatively homogeneous. Its signal intensity on T1-weighted spin-echo images is low and slightly higher than that of the myometrium. On T2-weighted spin-echo images, placental tissue has high signal intensity, and it is clearly distinguishable from the adjacent fetus, uterus, and cervix. Sagittal images best demonstrate the placental position in relation to the internal cervical os. Occasionally, endometrial veins can be seen at the margins of the placenta. Normal physiologic placental calcifications, which occur during late pregnancy, usually are not seen on MRIs.
Placenta previa is diagnosed when placental tissue covers all or part of the internal cervical os.
Degree of Confidence
To the authors' knowledge, no large prospective studies have been performed to examine the accuracy of MRI in the diagnosis of placenta previa. Several series have shown that results are similar and possibly slightly better than those of ultrasonography.
False Positives/Negatives
The authors did not find reports of false-negative results in the literature, and only a few accounts of false-positive findings have been reported. False-positive findings may result from myometrial contraction in the lower uterine segment at imaging. Although the placental margin remains distinct from the contracted muscle and the internal cervical os, the distance between the placental margin and the os may decrease, leading to false diagnosis of a low-lying placenta. In extreme cases, the edge of the placenta might come into contact with or even overlie a portion of the internal cervical os and thereby mimic placenta previa.
Findings
If the internal cervical os can be visualized and if no placental tissue overlies it, placenta previa is excluded. However, an attempt must be made to identify the inferior-most aspect of the placenta and to determine the distance between it and the internal os. When the fetal head obscures a posteriorly positioned placenta or when the inferior placental margin is not visualized at transabdominal imaging, a transvaginal or transperineal approach is nearly always adequate in revealing its position.
Although criteria may vary among institutions, any of the following findings excludes placenta previa: direct apposition of the presenting part of the fetus and the cervix without space for interposed tissue; amniotic fluid between the presenting part of the fetus and the cervix, without the presence of placental tissue; and a distance of greater than 2 cm between in the inferior aspect of the placenta and the internal cervical os on direct visualization.
Degree of Confidence
With a qualified operator, sonography is more than 95% accurate. Transvaginal evaluation of the placenta has a 1% false-positive rate and a 2% false-negative rate. Hertzberg and colleagues reported a 100% negative predictive value for transperineal sonography in a study of 164 patients.
False Positives/Negatives
The most common conditions that cause a false diagnosis of placenta previa are an overdistended bladder and myometrial contractions. Overdistension of the maternal urinary bladder places pressure on the anterior aspect of the lower uterine segment, compressing it against the posterior wall and causing the cervix to appear elongated. Thus, a normal placenta can appear to overlie the internal os. The cervix should be no longer than 3-3.5 cm during the third trimester. If the cervical length exceeds 3.5 cm or if a falsely elongated cervix is suspected, further imaging should be performed after the patient empties her bladder. Because transvaginal and transperineal imaging is performed when the patient's bladder is empty, this pitfall should be rare.
During a myometrial contraction, 2 situations that mimic placenta previa may occur: First, the wall of the uterus may thicken and imitate placental tissue. Second, the lower uterine segment can shorten and bring the inferior edge of the placenta into contact with the internal cervical os, creating a condition that imitates placenta previa. To avoid this pitfall, a contraction should be suspected if the myometrium is thicker than 1.5 cm. Findings from repeat imaging performed after 30 minutes should be sufficient to exclude this condition.
Finally, care should be taken when diagnosing placenta previa during the second trimester. This condition is reported to be 10-100 times as common in the second trimester as it is at term. Although the underlying physiology remains somewhat controversial, the disparity remains a fact. When placental tissue lies near or over the internal os in the second trimester, repeat imaging should be performed at a later date to confirm the diagnosis.
Findings
Although angiography has been used to diagnose placenta previa in the past, it no longer plays a role in this clinical situation.
In the case of pure placenta previa, that is, placenta previa not complicated by placenta accreta, the radiologist nearly always plays a solely diagnostic role. Occasionally, the interventionalist may be called upon to embolize the uterine arteries after cesarean delivery. If hemorrhage cannot be controlled surgically, this procedure may obviate emergency hysterectomy.
Medical/Legal Pitfalls
- Undiagnosed placenta previa may result in grave consequences during the latter trimesters and at the time of delivery.
- The relatively high incidence of placenta previa during the second trimester should not lead the radiologist to underdiagnose this condition.
- Whenever the diagnosis is suspected in the second trimester, further evaluation is recommended. In nearly all cases, this evaluation involves repeat sonography during the third trimester.
- Care should also be taken not to mistake a more serious situation, such as placental abruption or placenta accreta, for placenta previa, because the management of these conditions is different.
- In addition, the radiologist must avoid satisfaction-of-search errors.
- The possibility of one of these diagnoses complicating placenta previa must be excluded.
Special Concerns
- The likelihood of congenital anomalies and transverse fetal positioning is slightly higher in patients with placenta previa than in others.
- Special care should be taken to document such findings.
| Media file 1:
Longitudinal transabdominal sonogram demonstrates complete symmetric placenta previa. |
 |  View Full Size Image | |
Media type: Image
|
| Media file 2:
Ultrasonogram shows asymmetric complete placenta previa. Follow-up examination should be performed after the patient has voided to prevent false-positive results. An overdistended bladder can create the appearance of placenta previa. This finding is more common when the placenta is anterior. |
 | View Full Size Image | |
Media type: Image
|
| Media file 3:
Postvoiding longitudinal sonogram shows that the findings in a patient are not related to an overdistended bladder, but rather, to a true placenta previa. |
 | View Full Size Image | |
Media type: Image
|
| Media file 4:
Sagittal T2-weighted image (SSFSE) depicts a complete placenta previa in this 28-week pregnancy. |
 | View Full Size Image | |
Media type: MRI
|
- American College of Radiology. Role of imaging in second and third trimester bleeding. In: ACR Appropriateness Criteria. Reston, Va: American College of Radiology;2001.
- Ananth CV, Smulian JC, Vintzileos AM. The association of placenta previa with history of cesarean delivery and abortion: a metaanalysis. Am J Obstet Gynecol. Nov 1997;177(5):1071-8. [Medline].
- Artis AA 3rd, Bowie JD, Rosenberg ER, Rauch RF. The fallacy of placental migration: effect of sonographic techniques. AJR Am J Roentgenol. Jan 1985;144(1):79-81. [Medline].
- Benirschke K, Kaufmann P. Pathology of the Human Placenta. 2nd ed. New York, NY: Springer-Verlag;1990.
- Bhide A, Prefumo F, Moore J, et al. Placental edge to internal os distance in the late third trimester and mode of delivery in placenta praevia. BJOG. Sep 2003;110(9):860-4. [Medline].
- Bowie JD, Andreotti RF, Rosenberg ER. Sonographic appearance of the uterine cervix in pregnancy: the vertical cervix. AJR Am J Roentgenol. Apr 1983;140(4):737-40. [Medline].
- Bowie JD, Rochester D, Cadkin AV, et al. Accuracy of placental localization by ultrasound. Radiology. Jul 1978;128(1):177-80. [Medline].
- Dashe JS, McIntire DD, Ramus RM, et al. Persistence of placenta previa according to gestational age at ultrasound detection. Obstet Gynecol. May 2002;99(5 Pt 1):692-7. [Medline].
- Gillieson MS, Winer-Muram HT, Muram D. Low-lying placenta. Radiology. Aug 1982;144(3):577-80. [Medline].
- Hertzberg BS, Bowie JD, Carroll BA, et al. Diagnosis of placenta previa during the third trimester: role of transperineal sonography. AJR Am J Roentgenol. Jul 1992;159(1):83-7. [Medline].
- Hertzberg BS, Bowie JD, Weber TM, et al. Sonography of the cervix during the third trimester of pregnancy: value of the transperineal approach. AJR Am J Roentgenol. Jul 1991;157(1):73-6. [Medline].
- Iyasu S, Saftlas AK, Rowley DL, et al. The epidemiology of placenta previa in the United States, 1979 through 1987. Am J Obstet Gynecol. May 1993;168(5):1424-9. [Medline].
- Leerentveld RA, Gilberts EC, Arnold MJ, Wladimiroff JW. Accuracy and safety of transvaginal sonographic placental localization. Obstet Gynecol. Nov 1990;76(5 Pt 1):759-62. [Medline].
- Levine D, Barnes PD, Edelman RR. Obstetric MR imaging. Radiology. Jun 1999;211(3):609-17. [Medline].
- Levine D, Edelman RR. Fast MRI and its application in obstetrics. Abdom Imaging. Nov-Dec 1997;22(6):589-96. [Medline].
- McShane PM, Heyl PS, Epstein MF. Maternal and perinatal morbidity resulting from placenta previa. Obstet Gynecol. Feb 1985;65(2):176-82. [Medline].
- Nagayama M, Watanabe Y, Okumura A, et al. Fast MR imaging in obstetrics. Radiographics. May-Jun 2002;22(3):563-80; discussion 580-2. [Medline].
- Powell MC, Buckley J, Price H, et al. Magnetic resonance imaging and placenta previa. Am J Obstet Gynecol. Mar 1986;154(3):565-9. [Medline].
- Rosati P, Guariglia L. Clinical significance of placenta previa detected at early routine transvaginal scan. J Ultrasound Med. Aug 2000;19(8):581-5. [Medline].
- Sanderson DA, Milton PJD. The effectiveness of ultrasound screening at 18-20 weeks' gestational age for prediction of placenta previa. J Obstet Gynaecol. 1991;11:320.
- Scott JR. Placenta previa and abruption. In: Danforth DN, ed. Obstetrics and Gynecology. 8th ed. Philadelphia, Pa: Lippincott, Williams, & Wilkins;1999: 407-18.
- Sorokin Y. Obstetric hemorrhage. In: Ransom SB, ed. Practical Strategies in Obstetrics and Gynecology. Philadelphia, Pa: WB Saunders;2000: 311-20.
- Spirt BA, Gordon LP. Sonogrpahic evaluation of the placenta. In: Rumack CM, ed. Diagnostic Ultrasound. 2nd ed. New York, NY: Mosby-Year Book;1998: 1337-58.
- Thorp JM Jr, Councell RB, Sandridge DA, Wiest HH. Antepartum diagnosis of placenta previa percreta by magnetic resonance imaging. Obstet Gynecol. Sep 1992;80(3 Pt 2):506-8. [Medline].
- Townsend RR, Laing FC, Nyberg DA, et al. Technical factors responsible for "placental migration": sonographic assessment. Radiology. Jul 1986;160(1):105-8. [Medline].
- Townsend RR. Ultrasound evaluation of the placenta and umbilical cord. In: Callen PW. Ultrasonography in Obstetrics and Gynecology. 3rd ed. Philadelphia, Pa: WB Saunders;1994: 440-65.
- Worthington BS, Yuh WTC, Stark DD, et al. Obstetrics. In: Stark DD, ed. Magnetic Resonance Imaging. 3rd ed. St Louis, Mo: Mosby-Year Book;1999: 591-615.
- Zemlyn S. The effect of the urinary bladder in obstetrical sonography. Radiology. Jul 1978;128(1):169-75. [Medline].
Placenta Previa excerpt Article Last Updated: Aug 26, 2005
|
