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Am Fam Physician. 2007;76(8):1192-1194

Clinical Question

Which patients are likely to have a successful vaginal birth after a previous cesarean delivery?

Evidence Summary

The American College of Obstetricians and Gynecologists and the American Academy of Family Physicians recommend that pregnant women with a single previous cesarean delivery and a low-transverse incision be offered a trial of labor.1,2 Although the rate of vaginal birth after cesarean delivery (VBAC) increased from 19 percent of all deliveries in 1989 to a peak of 28 percent in 1996, the rate decreased to only 9.2 percent of all deliveries in 2004.3,4 The decline has been partially driven by concerns about the small but measurable risk of uterine rupture with VBAC, particularly when labor is induced or augmented.5

At the same time, the total number of cesarean deliveries has been increasing, largely because of an increase in primary cesarean deliveries.3 A screening tool to help predict whether a woman will have a successful VBAC may help patients and their physicians make more informed shared decisions.

A 2003 evidence review by the Agency for Healthcare Research and Quality found overall VBAC success rates between 60 and 82 percent in published studies, with an estimated overall success rate of 75 percent at teaching institutions and tertiary medical centers. The risks of perinatal death or hysterectomy from uterine scar rupture were low (1.5 and 4.8 per 10,000 births, respectively).4 The review identified factors associated with an increased likelihood of vaginal delivery (i.e., maternal age younger than 40 years, previous successful vaginal delivery, and favorable cervical factors). The review also identified factors that decreased the likelihood of vaginal birth (i.e., more than one previous cesarean delivery; induction of labor; birth weight greater than 4,000 g [8 lb, 13 oz]; and gestational age greater than 40 weeks).4

A number of researchers have attempted to develop clinical decision rules to predict the likelihood of a successful trial of labor after a previous cesarean delivery. A 2004 systematic review identified six of these clinical decision rules,6 two of which were validated (i.e., tested in a new population to confirm accuracy of the rule).7,8 Three subsequent rules were developed and validated.911 These five validated rules are summarized in Table 1.711

StudyPopulation characteristicsNumber of patientsVariablesType of rule
Development groupValidation group
Troyer, et al., 19928 Previous low-transverse cesarean delivery, gestational age > 36 weeks264236No previous vaginal birth, nonreassuring fetal heart tone at admission, induction of labor, previous dysfunctional laborAdditive point score
Flamm, et al., 19977 Previous low-transverse cesarean delivery, gestational age ≥ 37 weeks2,5022,501Maternal age, history of vaginal birth, a reason other than failure to progress for first cesarean delivery, cervical effacement at admission, cervical dilation ≥ 4 cm at admissionAdditive point score
Smith, et al., 200510 Single previous low-transverse cesarean delivery, gestational age ≥ 40 weeks11,64311,643Maternal height, maternal age, previous vaginal birth, gestational age, method of induction, sex of infantMultiplicative score
Grobman, et al., 200711 Single previous low-transverse cesarean delivery, gestational age ≥ 37 weeks5,9285,928Maternal age, body mass index, race, vaginal birth since last cesarean delivery, previous vaginal birth, nonrecurrent indication*Logistic regression equation and nomogram
Hashima, et al., 20079 Single previous low-transverse cesarean delivery5,4145,414Recurrent indication,* no history of fetal macrosomia, no current maternal anemiaAdditive point score

Although the Troyer rule was validated, the number of patients in the validation group was small.8 The Hashima rule was also prospectively validated; however, only three out of 5,414 women had a score of 0, and only 101 had a score of 1 (low probability of success).9 The remaining 5,310 women had scores of 2 (53 percent success rate) or 3 (67 percent success rate), which provides little useful information for decision making.9

The remaining three scores were well validated and were shown to be accurate in a large, representative population.7,10,11 The Flamm rule (Table 2) is the simplest to use, although it is limited by its age (data were gathered between 1990 and 1992) and by the requirement of cervical effacement information, which makes it unhelpful for antepartum planning.7

Patient characteristicsPoints
Maternal age < 40 years2
Vaginal birth history (choose one)
Vaginal birth before and after first cesarean delivery4
Vaginal birth after first cesarean delivery2
Vaginal birth before first cesarean delivery1
No previous vaginal birth0
Reason other than failure to progress for first cesarean delivery1
Cervical effacement at admission
> 75 percent2
25 to 75 percent1
< 25 percent0
Cervical dilation ≥ 4 cm at admission1

The Smith rule is well validated but is based on a multivariate equation, making it too complex for practical use at the point of care.10 Although the Grobman rule is also based on a complex multivariate equation, a nomogram (Figure 1) is provided for use at the point of care.11 The Grobman rule has been well validated and all of the needed variables are available to the patient and physician before the onset of labor.

Applying the Evidence

A 25-year-old, non-Hispanic, white woman with a body mass index (BMI) of 25 kg per m2 is in labor. Her cervix is 3 cm dilated and about 30 to 40 percent effaced. She has had one previous pregnancy, which resulted in cesarean delivery because of failure to progress. She wonders how likely it is that a trial of labor will be successful.

Answer: Using the Flamm rule (Table 27 ), the patient receives two points for age and one for cervical effacement. The total score of 3 gives her a 60 percent probability of vaginal delivery. Using the Grobman nomogram (Figure 111 ), the patient receives 10 points for age, 30 for body mass index, 7 points each for being non–African American and non-Hispanic, and 0 for no history of vaginal birth or recurrent primary indication. The total score of 54 points gives her a 68 percent probability of vaginal delivery. You advise the patient that her chance of a successful trial of labor is about two out of three.

This guide is one in a series that offers evidence-based tools to assist family physicians in improving their decision-making at the point of care.

This series is coordinated by Mark H. Ebell, MD, MS, deputy editor for evidence-based medicine.

A collection of Point-of-Care Guides published in AFP is available at

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