by Henci Goer

Commentary on:  Smith GCS. et al. Risk of perinatal death associated with labor after previous cesarean delivery in uncomplicated term pregnancies. JAMA 2002;287(20):2684-2690. [Abstract]

Study design and results: evaluation of 313,238 births, excluding preterm births, breech babies, and malformed babies. The investigators state that a study shows that the databases from which they extracted data were more than 98% accurate. The study recommends elective repeat cesarean at the 39th week of pregnancy.

• Death rates in babies during labor and up to 4 weeks after birth:
  • trial of labor (TOL): 13 per 10,000 (20/15,515); deaths possibly related to uterine scar rupture: 5 per 10,000 (8/15,515) Note: This is my calculation based on study data.
  • planned repeat cesarean section: 1 per 10,000 (1/9014) first-time mothers: 10 per 10,000 (135/137,160) prior births, no prior cesarean sections: 6 per 10,000 (90/151,549) 3/4 of the TOL group birthed vaginally

Problems include but are not limited to the following:

• The authors defined trial of labor as any vaginal birth or emergency cesarean after 37 weeks: This means that any woman who had a uterine rupture or placental abruption during pregnancy and presented at the hospital requiring an emergency cesarean would be classified as a TOL. The mortality in such cases would be high because these life-threatening events would occur outside of the hospital. To get a fix on this, another large study tells us that the odds of uterine rupture in pregnancy in a woman with a uterine scar are 2 per 1,000, and the odds of placental abruption are 3 per 1,000.(1) While not every baby would die, and some cases would occur earlier than 37 weeks, still, do the math on the 15,500 so-called TOLs, and you will see that some of the intrapartum and newborn deaths in the TOL group probably weren't really TOLs. Moreover, if any deaths in the emergency cesarean group were in women planning repeat cesarean, not only would the misclassification artificially inflate the death rate attributed to planned VBAC, it would artificially reduce the death rate in the planned cesarean group.
  
  
• Only 8 of the 20 perinatal deaths in the TOL group were possibly related to the uterine scar giving way: The investigators reported cause of death according to a hierarchy: toxemia, hemorrhage, mechanical, maternal, intrapartum anoxia, other pediatric diagnoses, and unexplained. Scar rupture fell into the “mechanical” category along with other mechanically based complications such as cord prolapse and birth injury. If a death involved more than one category, say, for example, a baby died from preeclampsia complicated by placental abruption, the cause of death would be assigned to the higher category, in this case, toxemia, rather than to hemorrhage or anoxia. Eleven of the 20 deaths were assigned to categories below “mechanical,” which means that scar rupture was not involved. A twelfth was assigned to toxemia. Whatever the reasons for these 12 deaths (issues of labor management come to mind), choosing to labor with a scarred uterus was not one of them.
  
  
• The absolute difference in mortality is small: Even without adjusting for the 12 deaths not associated with scar rupture, the excess mortality rate, that is, the difference in mortality between planned VBAC and planned repeat cesarean, was 12 per 10,000. The excess mortality rate attributable to having amniocentesis is 60 per 10,000.(2) Note too that the mortality rate with planned VBAC did not differ statistically from the mortality rate in first-time mothers.
  
  
• The study fails to consider long-term consequences of repeat cesarean (3): For the mother, surgery exposes her to increased likelihood of experiencing chronic pelvic pain, and bowel obstruction due to adhesions. The probabilities of these will almost certainly rise with additional surgeries. In addition, increasing amounts of pelvic scar tissue makes future surgeries more technically difficult for the surgeon and operative injury more likely. Babies developing in a cesarean-scarred uterus are more likely to be ectopic pregnancies (the embryo implants outside of the uterus), to die in late pregnancy, or to be born preterm, low birth-weight, or have a congenital anomaly. We do not know if having multiple cesareans increases the chances of perinatal mortality and morbidity, but it stands to reason that it would if uterine scarring is the root cause. Studies also show that accumulating cesarean scars increases the likelihood of placenta previa (the placenta overlays the cervix) and of having placenta accreta (the placenta grows into or through the uterine muscle) in conjunction with placenta previa. Placenta accreta is particularly deadly for mothers because it almost invariably causes massive hemorrhage.(4) The incidence of placenta accreta has increased 10-fold—from 1 in 25,000 deliveries to 1 in 2500 deliveries—over the last 50 years, a rise attributed to the rise in cesarean surgery.(5)

Comment: The investigators recommend repeat cesarean section at 39 weeks, yet 3 out of 4 women in this study had vaginal births, effectively ending their exposure to the hazards of cesarean section. Nearly all women who birth vaginally after a previous cesarean will continue safely to have vaginal births should they have more children.(4,6)

Bibliography:

1. Rageth JC, Juzi C, and Grossenbacher H. Delivery after previous cesarean: a risk evaluation. Swiss Working Group of Obstetric and Gynecologic Institutions. Obstet Gynecol 1999;93 (3):332-7.
2. Seeds JW. Diagnostic mid trimester amniocentesis: how safe? Am J Obstet Gynecol 2004;191:608-16.
3. Childbirth Connection. Should I choose VBAC or repeat c-section? Available online at http://www.childbirthconnection.org/article.asp?ClickedLink=293&ck=10212&area=27
4. Lieberman E. Risk factors for uterine rupture during a trial of labor after cesarean. Clin Obstet Gynecol 2001;44(3):609-21.
5. ACOG. Placenta accreta. Committee Opinion No. 266, Jan 2002.
6. Hashima JN, Eden KB et al. Predicting vaginal birth after cesarean delivery: a review of prognostic factors and screening tools. Am J Obstet Gynecol 2004;190:547-55.