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ABSTRACT & COMMENTARY

Risk of Uterine Rupture or Placenta Previa after Previous Myomectomy or Classical Cesarean

March 1, 2013
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By John C. Hobbins, MD, Professor of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora, is Associate Editor for OB/GYN Clinical Alert.

Dr. Hobbins reports no financial relationships relevant to this field of study.

Synopsis: A recent study shows that having a previous myomectomy or classical cesarean section does not convey a significantly greater risk of uterine rupture or placenta previa during a subsequent pregnancy, when compared with a low transverse section.

Source: Gyamfi-Bannerman C, et al. Risk of uterine rupture and placenta accreta with prior uterine surgery outside of the lower segment. Obstet Gynecol 2012;120:1332-1337.

Conventional wisdom in obstetrics implies that woen with previous uterine surgery outside the lower uterine segment — whether myomectomy or classical cesarean section — incrue an unacceptable risk for uterine rupture during pregnancy. Recently, members of the NICHD perinatal network revisited this concept — this time with enough numbers to question this thinking.1

The group compared outcomes from three groups: 1) 167 women who had had prior myomectomies (PMM), 2) 455 pregnant women who had prior “classical” cesarean sections (PC), and 3) 1373 patients who had prior low transverse cesarean sections (LTCS). The dependent variables were uterine rupture (through the myometrium and uterine serosa) and documentation of placenta accreta.

The average gestational ages at delivery were: 35.8 weeks for PMM, 37.3 weeks for PC, and 38.5 weeks for LTCS. The uterine rupture incidence was 0% for the PM group, 0.88% for the PC group, and 0.41% for the LTCS patients. None of these differences were statistically significant. Placenta accreta occurred in none of the PMM, in 0.88% of the PC, and in 0.19% of the LTCS groups. The risk of accreta rose appreciably in patients who had placenta previa, but if present, there was little difference in the frequency of accreta between PC (13.6%) and the LTCS group (11.1%).

Commentary

The take-home points from this study are:

  1. Previous myomectomy has a very low risk of uterine rupture — at least up until 36 weeks.
  2. A previous classical cesarean section also conveys a very low risk of uterine rupture, but there was a trend toward a higher rate of placenta accreta (0.88% vs 0.19% in LTCS).
  3. In all groups, the risk of rupture and accreta (in those without placenta previa) was very low (< 1%).

One limitation of the study was that the authors could not retrieve data regarding how often the uterine cavity was entered during the myomectomies or the number and location of the myomas. Another interesting deficiency was that the myomectomy pregnancies all were delivered by cesarean section at an average gestational age of 35.8 weeks and, since only 20% of these patients were reported to have had regular uterine contractions, we do not know what would have happened if these pregnancies had continued to term.

It is surprising that the term “classical” cesarean section was not specifically defined in the methods section. The original description of a classical cesarean section pertained to those that were done years ago when the uterine incision involved the upper segment and fundus. Since there were so many PC patients included in the study, it is probably safe to assume that the term “classical” was synonymous simply with a vertical incision somewhere in the uterus.

Other than a few case reports, it is unclear why a history of a PMM has been stigmatized to a point where clinicians automatically default to an early term “prophylactic” cesarean section. This study showed a 0% chance of rupture in these patients, and these results are consistent with another study involving 1225 pregnant women with previous myomectomies who had a rupture rate of only 0.24%.2

Prior vertical incisions also have been treated with great caution, another stigma that may be undeserved. For example, in this study only 5.9% in the PC group delivered vaginally (compared with 44% in the LTCS group), strongly suggesting a reluctance to take these patients to term or even to let them labor. Yet, the uterine rupture rate in this group was not statistically different than that of the LTCS group, with both being < 1%. The somewhat greater risk of accreta in the PC group could simply mean that an anterior placenta covers more scar surface than it would with a LTCS scar, especially if there was no evidence of placenta previa.

The findings of this study should send a message that the idea of “prophylactic” early-term cesarean section in patients who simply have a history of a previous myomectomy needs rethinking. If not immediately abandoned, this method should at least be studied in comparison with expectant management.

References

  1. Gyamfi-Bannerman C, et al. Risk of uterine rupture and placenta accrete with prior uterine surgery outside of the lower segment. Obstet Gynecol 2012;120:1332-1337.
  2. Obed JY, Omigbodun A. Rupture of the uterus in patients with previous myomectomy and primary caesarean section scars: A comparison. J Obstet Gynecol 1996;16:16-21.