American Journal of Obstetrics & Gynecology
Volume 201, Issue 3 , Pages 227-229, September 2009

A new classification of placenta previa: Measuring progress in obstetrics

  • Lawrence W. Oppenheimer, MD, FRCOG, FRCSC

      Affiliations

    • Division of Maternal Fetal Medicine, University of Ottawa, Ottawa Hospital General Campus, Ottawa, Ontario, Canada
    • ,
  • Dan Farine, MD, FRCSC

      Affiliations

    • Division of Maternal Fetal Medicine, University of Toronto, Mount Sinai Hospital, Toronto, Ontario, Canada

Article Outline

 

Old obstetric dogmas can take a long time to debunk. Witness castor oil, enemas, and lying-in hospitals, to name but a few. So it is with the traditional classification of placenta previa. The original description of placenta previa is credited to Portal in 1683, although Schacher in 1709 was the first to demonstrate postmortem the exact relationship of the placenta to the uterus.1 The classification of placenta previa into complete, partial, and marginal probably had its origins in the 19th century. The description was meant to refer to the extent to which the placenta could be palpated through the cervix.1 Complete previa referred to an implantation over the internal os where the margin of the placenta could not be felt; partial previa referred to the placenta covering a closed internal os, but not completely covering a dilated os; and marginal previa meant an implantation in which the margin could be easily felt. Lateral (or low-lying) previa is the variety in which the margin of the placenta can only be felt with difficulty. In the United Kingdom, the description of placenta previa into grades I-IV or major and minor has been used commonly.

See related article, page 266

The distinction between placental abruption and previa was based on the ability to palpate the placenta through the cervical os; the difference was important because the treatment of previa involved rupture of the membranes, internal podalic version, and use of the fetus as a tamponade! The realization that digital palpation might not be such a good idea, and the introduction of conservative management with blood transfusion by MacAfee2 in 1945, lead to the need for a more accurate diagnosis. Imaging modalities to investigate placental location were introduced after the advent of radiology. In the 1930s amniography and cystography were explored. Gottesfeld et al3 introduced the use of ultrasound for placental location in 1966, and the first description of vaginal sonography, attributed to Kratochwil, followed in 1969.4 Transvaginal sonography (TVS) for the diagnosis of placenta previa has become the gold standard.5 Transabdominal ultrasound is inaccurate in the diagnosis of previa and should be used only as a screening tool.6 TVS is safe, even in the presence of active bleeding.7, 8 The accurate localization of the placental edge in relation to the discrete point of the internal os by TVS makes the use of the terms marginal, partial, and low-lying outmoded (Figure). What the clinician really wants to know to guide treatment is the likelihood of antepartum hemorrhage and need for cesarean section delivery, based on the exact distance from the cervix. There is now a growing literature on this relationship.9, 10, 11, 12, 13 A placental edge lying <2 cm away from the internal os on TVS has become generally accepted as the threshold for the performance of cesarean section delivery for previa at term. An inherent problem in all the published studies to date is the likelihood that knowledge of the distance itself may have lead to the decision to perform the cesarean section delivery, rather than the clinical features of the case. In this respect, the contribution by Vergani et al14 in this edition of the Journal is valuable. Although also a retrospective study, the authors describe a policy of expectant management in the largest series to date of 53 women with a cephalic presentation and a placental edge–to–os distance on TVS between 1-20 mm. Cases were divided into 2 groups: 1-10 mm from the os (n = 24 cases) and 11-20 mm (n = 29 cases). They found a cesarean section delivery rate of 75% and 31%, respectively, and an incidence of antepartum hemorrhage of 29% vs 3%, respectively. The scans were all performed within 28 days of delivery at a mean gestational age of 36.4 weeks, and delivery occurred on average 10 days later. None of the 11-20 mm group required cesarean section delivery for antepartum hemorrhage, and none required cesarean section delivery in labor. They conclude that women with a placenta that is situated 11-20 mm away can be offered a trial of labor. The data of Vergani corroborates well with the 2 other publications that have reported the same distance groups.10, 11 Pooling the 3 data sets gives a cesarean section delivery rate of 78% (17/50 cases) for a distance of 0-10 mm and 34% (39/50 cases) for 11-20 mm.

  • View full-size image.
  • FIGURE. 

    Transvaginal sonogram

  • The tip of the probe is located at the top of the picture. The cervical canal is seen in the upper half of the image, and a posterior placenta is seen in the lower half of the image, with the placental edge lying 7 mm away from the internal cervical os. Part of the fetal head is seen on the left side.

  • Oppenheimer. A new classification of placenta previa. Am J Obstet Gynecol 2009.

Vergani et al propose that the time-honored classification of placenta previa should be abandoned. We agree with them and others who have published the same sentiments.15, 16 Admittedly, the data is imperfect. The numbers of cases that have been reported is still small and are based only on retrospective studies, although it might be difficult to mount a trial in which the obstetrician is blinded to the exact location of the placenta.

We need more information on the likelihood of antepartum hemorrhage based on placental edge distance and the safety of out-patient treatment.17 Treatment decisions should be based on the measured distance of the placental edge to the internal cervical os by transvaginal ultrasound whenever possible. The routine reporting of this distance will enable us to confirm the current assumptions rapidly. Recognizing that measurements of <1 cm may be subject to error and operator variability, it probably makes sense to group the distance to the nearest centimeter.

A new classification could describe the distance on TVS that is performed within 28 days of term in the following way: (1) >20 mm away from the internal os; cesarean section delivery for previa not indicated; (2) 11-20 mm; lower likelihood of bleeding and need for cesarean section delivery; (3) 0-10 mm; higher likelihood of bleeding and need for cesarean section delivery; and (4) overlap of the internal os by any distance: cesarean section delivery indicated.

The distance alone should not be a replacement for clinical judgment in regard to factors such as unstable lie or significant antepartum hemorrhage. As more data accumulates, we can add better estimates of the risk of bleeding before and during labor and the likelihood of successful vaginal delivery. We still have 4 groups, but the description makes a lot more sense. The education exercise really has to start not just in the obstetric domain but with the sonographers and physicians who perform and report obstetric ultrasound.

Approximately 3% of the obstetric population in the second trimester will have a placental edge low enough to justify follow up with transvaginal ultrasound.18 The study by Vergani et al also allows an estimate of the incidence of a placenta lying within 2 cm of the os at 36 weeks of gestation at approximately 0.6%, one-half of whom will have a placental edge overlapping the internal os and a similar number will have a placental edge of 1-20 mm away that will warrant a decision regarding treatment. The benefits of accurate diagnosis by TVS include risk assessment for outpatient treatment, selection for trial of labor, and screening for placenta accreta.19 In addition, exclusion of vasa previa, which is associated strongly with a placenta that is initially located in the lower segment,20 can also be achieved with color Doppler sonography. Investigation of antepartum hemorrhage by TVS should be routine whenever there is doubt about the exact placental location.

The capability to measure accurately placental location has been around for >20 years. All it will take to consign the old classification of placenta previa to the history books is a shift in our thinking by a couple of centimeters.

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References 

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PII: S0002-9378(09)00630-9

doi:10.1016/j.ajog.2009.06.010

Refers to article:

  • Editor's ChoiceEditor's CommentaryCross-referenceArticles in full Placenta previa: distance to internal os and mode of delivery , 27 July 2009

    Patrizia Vergani, Sara Ornaghi, Ilaria Pozzi, Pietro Beretta, Francesca Maria Russo, Ilaria Follesa, Alessandro Ghidini
    American Journal of Obstetrics & Gynecology September 2009 (Vol. 201, Issue 3, Pages 266.e1-266.e5)

American Journal of Obstetrics & Gynecology
Volume 201, Issue 3 , Pages 227-229, September 2009

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