Introduction
Presentation
Demographics
Incidence
- 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.1 Their results indicate that placenta previa was diagnosed in 0.48% of all births. This rate of incidence corresponds to rates reported in earlier, smaller studies.
- In the United States, the incidence of placenta previa is reported to be slightly higher in minority populations. Increased maternal age is a known risk factor for placenta previa.
- 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.2
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 is 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 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.3 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.
Presentation
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.4,5,6
The reported incidence of placenta previa in the second trimester is nearly 10 times that at delivery. Various 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, indicate that a complete placenta previa in the second trimester rarely reverts to a normal position at term.
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.
Historically, placenta previa was diagnosed by means of digital palpation of the placental tissue through the cervical canal. The slightest amount of manipulation, however, may 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.
Natural history
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.
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 may 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.7
Treatment
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.
Related eMedicine topic:
Placenta Previa (from Emergency Medicine)
Related Medscape topics:
Specialty Site Radiology
Radiology CME and News
Radiology Conferences
Preferred Examination
Differential Diagnoses
Other Problems to Be Considered
Overdistended bladder
Myometrial contraction
Placenta accreta
Vasa previa
Low-lying placenta
More on Placenta Previa |
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| Multimedia: Placenta Previa |
| References |
| Further Reading |
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References
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].
Yang Q, Wu Wen S, Caughey S, Krewski D, Sun L, Walker MC. Placenta previa: its relationship with race and the country of origin among Asian women. Acta Obstet Gynecol Scand. 2008;87(6):612-6. [Medline].
McShane PM, Heyl PS, Epstein MF. Maternal and perinatal morbidity resulting from placenta previa. Obstet Gynecol. Feb 1985;65(2):176-82. [Medline].
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.
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.
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].
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].
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].
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].
Masselli G, Brunelli R, Casciani E, Polettini E, Piccioni MG, Anceschi M, et al. Magnetic resonance imaging in the evaluation of placental adhesive disorders: correlation with color Doppler ultrasound. Eur Radiol. Jun 2008;18(6):1292-9. [Medline].
Dedes I, Ziogas V. Circular isthmic-cervical sutures can be an alternative method to control peripartum haemorrhage during caesarean section for placenta praevia accreta. Arch Gynecol Obstet. May 1 2008;[Medline].
American College of Radiology. Role of imaging in second and third trimester bleeding. In: ACR Appropriateness Criteria. Reston, Va: American College of Radiology;2001.
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].
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].
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].
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.
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.
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.
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].
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].
Choi SJ, Song SE, Jung KL, Oh SY, Kim JH, Roh CR. Antepartum risk factors associated with peripartum cesarean hysterectomy in women with placenta previa. Am J Perinatol. Jan 2008;25(1):37-41. [Medline].
Imarengiaye CO, Osaigbovo EP, Tudjegbe SO. Anesthesia for cesarean section in pregnancies complicated by placenta previa. Saudi Med J. May 2008;29(5):688-91. [Medline].
Further Reading
Placenta praevia and placenta praevia accreta: diagnosis and management.
Royal College of Obstetricians and Gynaecologists. 2001 Jan (revised 2005 Oct). 12 pages. NGC:004763
Second and third trimester bleeding.
American College of Radiology. 1996 (revised 2005). 2 pages. NGC:004652
Ultrasound scanning during pregnancy.
Finnish Medical Society Duodecim. 2000 Apr 3 (revised 2004 Jun 28). Various pagings. [NGC Update Pending] NGC:004106
Keywords
placenta previa, abnormal implantation, low implantation, internal cervical os, complete or total placenta previa, incomplete or partial placenta previa, marginal placenta previa, low-lying placenta previa
