Placenta Previa – Quick Consult
Last Updated / Reviewed: October 2024
Definition
Key History
Key Physical Exam
Risk Factors for Placenta Previa
Classification Systems
Differential Diagnosis
Diagnostic Testing
Clinical Risk and Safety Pearls
Treatment
Complications

Definition

Placenta previa is defined as implantation of the placenta over or near the internal os of the cervix. The condition occurs in 5 out of 1,000 deliveries.

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Key History

  • Bright red vaginal bleeding – painless, sudden, and may be heavy
  • Contractions
  • May have history of previous episodes of bleeding
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Key Physical Exam

  • Hypotension
  • Tachycardia
  • Profuse hemorrhage
  • Non-tender soft uterus
  • Normal fetal heart tones – typically
  • Do not perform vaginal or rectal exam – may provoke uncontrollable bleeding
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Risk Factors for Placenta Previa

  • Age over 30 – 3 times more likely
  • Prior previa
  • Pregnancy following a cesarean section
  • Multiparity
  • Multiple gestations
  • Prior induced abortion – sharp curettage
  • Smoking
  • Cocaine use
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Classification Systems

Placental Location Location Description
Total placenta previa The internal cervical os is completely covered by the placenta.
Partial placenta previa The internal os is partially covered by the placenta.
Marginal placenta previa The placenta is at the margin of the internal os.
Low-lying placenta previa The edge of the placenta is near the internal os but does not reach it.

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Differential Diagnosis

  • Vaginal bleeding – infectious, other
  • Vasa previa
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Diagnostic Testing

The recommended diagnostic tests for placenta previa primarily involve imaging studies, with ultrasound being the cornerstone of diagnosis.
  • Transabdominal Ultrasound: This is the initial imaging modality used to evaluate the placenta's location, the inferior placental margin, and its relationship to the internal cervical os. It is effective for screening and can identify placenta previa with high sensitivity and specificity.
  • Transvaginal Ultrasound (TVUS): If the transabdominal ultrasound is inconclusive or does not adequately visualize the internal cervical os and inferior placental margin, a transvaginal ultrasound is recommended. TVUS provides a more detailed and accurate assessment of the placental location and is safe to use in cases of suspected placenta previa.
  • Transperineal Ultrasound: This technique can be used as an adjunct to transabdominal ultrasound, especially in the third trimester when the cervix may be difficult to visualize. It offers an additional view of the cervix and can help confirm or exclude placenta previa.
  • Magnetic Resonance Imaging (MRI): While not routinely required, MRI can be used in complex cases or when there is suspicion of placenta accreta spectrum disorders. MRI provides detailed imaging of the placenta and its relationship to surrounding structures, although it is generally reserved for cases where ultrasound findings are inconclusive or when there is a high suspicion of placental invasion.
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Clinical Risk and Safety Pearls

  • Do not perform vaginal or rectal exam – may provoke uncontrollable bleeding.
  • Emergency obstetric consult.
  • Apply fetal monitoring as soon as possible.
  • Carefully document the course of events, medical decision-making, and communications with consultants.
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Treatment

Common text / literature recommendations include:
  • Asymptomatic placenta previa:
    • Monitor the previa for resolution with increasing gestational age.
    • Reduce the risk of bleeding.
    • If the previa persists, determine the optimal time for planned C-section.
  • Acute care of bleeding placenta previa: The major goal is achieving or maintaining maternal hemodynamic stability and determining if an emergency C-section is indicated.
    • Monitor both maternal and fetal vital signs.
    • Use visual aids to quantify the amount of bleeding (e.g., maternity pad, emesis basin, etc.).
    • CBC, type and crossmatch, and notify the blood bank that a patient with placenta previa has been admitted; evaluate for coagulopathy.
    • Cesarean delivery is indicated for:
      • Active labor
      • A nonreassuring fetal heart tracing unresponsive to resuscitative measures
      • Severe and persistent vaginal bleeding, such that hemodynamic stability cannot be achieved or maintained
      • Significant vaginal bleeding at 34 weeks of gestation
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Complications

  • Placenta accreta*
  • Vasa previa**
  • Rebleeding
  • Intrauterine growth retardation (IUGR) - 16% incidence
  • Congenital anomalies
  • Fetal anemia / Rh isoimmunization
  • Maternal mortality – rare
*Placenta accreta is a severe obstetric complication involving an abnormal superficial attachment of the placenta to the myometrium (the middle layer of the uterine wall). There are three forms of placenta accreta, distinguishable by the depth of penetration. The placenta usually detaches from the uterine wall relatively easily, but women who encounter placenta accreta during childbirth are at great risk of hemorrhage during its removal. This commonly requires surgery to stem the bleeding and fully remove the placenta, and in severe forms, can often lead to a hysterectomy or be fatal.

**Vasa previa is an obstetric complication defined as "fetal vessels crossing or running in close proximity to the inner cervical os.” These vessels course within the membranes (unsupported by the umbilical cord or placental tissue) and are at risk of rupture when the supporting membranes rupture.

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Alouini S, Megier P, Fauconnier A, et al. Diagnosis and management of placenta previa and low placental implantation. J Matern Fetal Neonatal Med. 2019:1-6.

American College of Obstetricians and Gynecologists. ACOG Committee Opinion No. 560: Medically indicated late-preterm and early-term deliveries. Obstet Gynecol. 2013;121:908-910. Reaffirmed 2017.

Anderson-Bagga FM, Sze A. Placenta Previa. [Updated 2023 Jun 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539818/.

Expert Panel on GYN and OB Imaging; Shipp TD, Poder L, Feldstein VA, Oliver ER, Promes SB, Strachowski LM, Sussman BL, Wang EY, Weber TM, Winter T, Glanc P. ACR Appropriateness Criteria® Second and Third Trimester Vaginal Bleeding. J Am Coll Radiol. 2020 Nov;17(11S):S497-S504. doi: 10.1016/j.jacr.2020.09.004. PMID: 33153560.

Fan D, Xia Q, Liu L, et al. The incidence of postpartum hemorrhage in pregnant women with placenta previa: A systematic review and meta-analysis. PLoS One. 2017;12(1):e0170194.

Jain V, Bos H, Bujold E. Guideline No. 402: Diagnosis and Management of Placenta Previa. J Obstet Gynaecol Can. 2020 Jul;42(7):906-917.e1. doi: 10.1016/j.jogc.2019.07.019. PMID: 32591150.

Jauniaux E, Alfirevic Z, Bhide AG, et al. on behalf of the Royal College of Obstetricians and Gynaecologists. Placenta praevia and placenta accreta: Diagnosis and management. Green-top Guideline No. 27a. BJOG. 2019;126(1):e1-e48.

Shin JE, Shin JC, Lee Y, Kim SJ. Serial change in cervical length for the prediction of emergency cesarean section in placenta previa. PLoS One. 2016;11(2):e0149036.

Silver RM. Abnormal placentation: Placenta previa, vasa previa, and placenta accreta. Obstet Gynecol. 2015;126(3):654-668.

Society for Maternal-Fetal Medicine (SMFM), Gyamfi-Bannerman C. Society for Maternal-Fetal Medicine (SMFM) Consult Series #44: Management of bleeding in the late preterm period. Am J Obstet Gynecol. 2018;218(1):B2-B8.

This is intended solely as reference material and is not a recommendation for any specific patient. The practitioner must rely upon his or her own professional judgment and medical decision-making to determine whether it is relevant in a particular case. Materials are derived from medical and nursing texts, medical literature and national guidelines and should not be considered complete or authoritative. Users must rely on specific patient presentation, experience and judgment when utilizing any of the information contained herein relative to an actual patient.
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