Great article on study showing risks decrease w/successive VBACs - Mothering Forums

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This is from the February issue of the Journal of Obstetrics and GYnecology. All italics and underlinings are my own.

Prior Successful Vaginal Birth After Cesarean Delivery Linked to Low Risk for Complications in Later Attempts CME/CE
News Author: Laurie Barclay, MD
CME Author: Désirée Lie , MD , MSEd
Release Date: February 8, 2008;
February 8, 2008 — Women with previous successful attempts at vaginal birth after cesarean delivery (VBAC) are at low risk for maternal and neonatal complications during subsequent VBAC attempts, according to the results of a study reported in the February issue of Obstetrics & Gynecology.
"The relationships between the number of prior VBACs and the probability of successful VBAC attempt or uterine rupture in the current pregnancy remain to be clearly elucidated," write Brian M. Mercer, MD, from Case Western Reserve University-MetroHealth Medical Center in Cleveland , Ohio , and colleagues from the National Institute of Child Health and Human Development Maternal–Fetal Medicine Units Network. "It is also unknown if successive labors will place an additive strain on the uterine scar, increasing the risk of uterine rupture when VBAC is attempted. The purpose of this analysis is to evaluate the impact of increasing number of prior VBACs on the likelihood of VBAC success and uterine rupture in subsequent pregnancies."
Using a prospective, multicenter registry encompassing 19 clinical centers from 1999 to 2002, the investigators selected 13,532 women with 1 or more previous low transverse cesarean deliveries who attempted VBAC in their current pregnancy. They compared outcomes based on the number of previous VBAC attempts after the last cesarean delivery.
With an increasing number of previous VBACs, the success of VBAC increased: 63.3%, 87.6%, 90.9%, 90.6%, and 91.6% for those with 0, 1, 2, 3, and 4 or more previous VBACs, respectively (P < .001). After the first successful VBAC, the rate of uterine rupture decreased, and it did not increase thereafter: 0.87%, 0.45%, 0.38%, 0.54%, 0.52%, respectively (P =.03), and the risk for uterine dehiscence and other peripartum complications also decreased. With an increasing number of VBACs thereafter, there was no increase in neonatal morbidities.
"Women with prior successful VBAC attempts are at low risk for maternal and neonatal complications during subsequent VBAC attempts," the study authors write. "An increasing number of prior VBACs is associated with a greater probability of VBAC success, as well as a lower risk of uterine rupture and perinatal complications in the current pregnancy."
Limitations of the study include limited power to adequately compare outcomes between individual subgroups; and evaluation of outcomes in mostly large urban tertiary care hospitals, creating possible bias.
"We believe that the findings of this prospective analysis of a large cohort of pregnancies will provide important information for counseling women who are considering their options regarding VBAC," the study authors conclude. "Our results are particularly important for those considering repeated pregnancies after an initial cesarean delivery. Although women planning large families should consider the risks associated with repeated cesarean deliveries, they should be reassured by the increasing success rates and decreasing risks associated with VBAC attempts in successive pregnancies."
The National Institute of Child Health and Human Development supported this study. The study authors have disclosed no relevant financial relationships.
Obstet Gynecol. 2008;111:285-291.
Learning Objectives for This Educational Activity
Upon completion of this activity, participants will be able to:

1. Describe the association between the success rate of vaginal births after cesarean delivery and the number of these previous births after low transverse cesarean delivery.
2. Describe the risks for uterine rupture, dehiscence, and other perinatal morbidities associated with the number of vaginal births after cesarean delivery.

Clinical Context
The frequency of cesarean delivery in the United States declined to 20.7% in 1996 and increased to 30.2% in 2005, with the majority (71%) being the first "primary" cesarean delivery. The likelihood of successful VBAC has been documented at 72%. The American College of Obstetrics and Gynecology has recently affirmed VBAC as an appropriate alternative to repeat cesarean delivery after a low transverse cesarean delivery, and the authors previously reported the odds ratio (OR) of successful VBAC after any previous vaginal delivery of 3.9 with an OR of 2.7 for 1 or more previous successful VBAC attempts. However, the risk for uterine rupture with successive VBAC attempts is controversial.
This is a secondary analysis of a 4-year observational study conducted at 19 academic centers of the National Institute of Child Health and Human Development Maternal–Fetal Medicine Units Network, to examine the success rate of increasing VBAC attempts and the associated intrapartum, peripartum, and perinatal risks.
Study Highlights

* Included were women with singleton pregnancies with at least 1 previous cesarean delivery by low transverse uterine incision with an attempted VBAC and delivery of an infant of at least 20 weeks' gestation or 500 g in the current pregnancy.
* Pregnancy outcomes evaluated included VBAC success, uterine rupture, dehiscence, thromboembolism, transfusion, endometritis, maternal death, neonatal intensive care admission, hypoxia, and infant death.
* Of 45,988 births with a singleton gestation and cesarean delivery, 13,532 women had a low transverse uterine incision and attempted VBAC with a success rate of 71.8%.
* In this study, mean maternal age was 28 to 30.6 years, 33.4% to 46.8% of women were African American, 38.0% to 40.1% were white, and 11.5% to 21.2% were Hispanic.
* Preterm birth rate was 12.8% to 15.5%, and mean pregravid body mass index was
26.5 kg/m2.
* Women with 0, 1, 2, 3, and 4 or more previous VBACs represented 66.6%, 21.4%, 7.8%, 2.7%, and 1.4% of the cohort, respectively.
* Maternal age, African American race, and government health insurance increased with increasing number of VBACs.
* The frequency of VBAC success rose with increasing number of previous VBACs, from 63.3% with no previous VBAC to 87.6% and 90.9% for those with 1, 2, or more VBACs, respectively (P < .001).
* The frequency of uterine rupture decreased from 0.87% with no previous VBAC to 0.45% and 0.43% for those with 1, 2, or more previous VBACs, respectively (P = .01).
* Risk for uterine dehiscence was inversely associated with number of previous VBACs at 0.94% vs 0.24% and 0.25% for those with 0, 1, or 2 or more previous VBACs, respectively (P < .001).
* The likelihood of VBAC success reached a plateau for those with 2 previous VBACs and did not increase thereafter (P < .001).
* Risk for uterine rupture declined after 1 successful VBAC and did not decline further with subsequent VBACs (P = .03).
* Results were similar for other maternal and neonatal outcomes.
* Other maternal morbidities and peripartum complications were less common with increasing number of previous VBACs, likely related to the decreased need for cesarean delivery.
* There were no increases in neonatal morbidities or mortality with increasing number of VBACs.
* The authors concluded that women who have had 1 or more previous VBACs can be reassured that the likelihood of vaginal delivery is increased and that the likelihood of complications is reduced after the first attempt.

Pearls for Practice

* An increasing number of previous VBACs are associated with an increased success rate of future VBACs, with a plateau after 2 previous VBACs.
* The rate of uterine rupture and other maternal complications is reduced after at least 1 successful VBAC.
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