TRANSVAGINAL ULTRASOUND IN PLACENTA PREVIA

 

ADVANTAGES

 

  • The risk of bleeding in experienced hands is negligible (1-4). The ultrasound probe is always inserted under direct visualization and is Never in contact with the external os.
  • Transvaginal ultrasound allows accurate visualization of the internal os in all cases. The exact distance from the internal os to the placental edge in cases of low-lying or marginal placenta previa (1).
  • Transvaginal sonography is invariable superior to transabdominal sonography in sensitivity, specificity, positive and negative predictive values (2).
  • A placental edge located two cm or more from the internal os is not associated with intrapartum hemorrhage (5).
  • Transvaginal ultrasound is useful in the diagnosis of placenta accreta (6) and vasa previa.

 

Placenta covers internal os and extends over the os by over 5 cm. This type will not migrate sufficiently by term and will likely require a cesarean section.

 

Placenta covers internal os and extends over the os by under 2 cm. This type will often migrate sufficiently by term and allow vaginal delivery.

 

 

 

 

 

Video clip of Complete Placenta Previa

 

 

 

DISA

DISADVANTAGES

 

  • False positive diagnosis of marginal placenta previa prior to 35 weeks gestation. Repeat scanning is recommended at two or three weeks intervals prior to elective caesarean section (3).
  • The entire uterine cavity cannot be visualized after 14 weeks gestation therefore transvaginal ultrasound needs to complement transabdominal scans.
  • Experience in endovaginal scanning is needed prior to performing this technique on patients with suspected placenta previa.

 

PITFALL IN THE DIAGNOSIS OF PLACENTA PREVIA

 

A prominent myometrial contraction in the lower uterine segment adjacent to the placenta may simulate a previa. The echotexture of the contaction is usually slightly different to the placenta, and resolution over time will usually help distinguish contraction from previa.

 

 

REFERENCES
  1. Farine D, Fox HE, Jacobson S et.al. Vaginal ultrasound for diagnosis of placenta previa. Am J Obstet Gynecol 1988;159:566-569.
  2. Leerentveld RA, Gilberts EC, Anrold MJ et.al. Accuracy and safety of transvaginal placental localization. Obstet Gynecol 1990;76:759.
  3. Sherman SJ, Carlson DE, Platt LD et.al. Transvaginal ultrasound: does it help in the diagnosis of placenta previa? Ultrasound Obstet Gynecol 1992;2(4):256-260.
  4. Pauzner D, Barrett J, Farine D. Transvaginal scanning in the management of placenta previa. J SOGC 1995;17:231-235.
  5. Oppenheimer LW, Farine D, Ritchie JWK et.al. What is a low lying placenta? Am J Obstet Gynecol 1991;165(4):1036-1038.
  6. Guy GP, Peisner DB, Timor Tritsch IE. Ultrasound evaluation of uteroplacental blood flow patterns of abnormally located and adherent placentas. Am J Obstet Gynecol 1990;163:723-727.