Clinical Articles The fetal-pelvic index: A method of identifying fetal-pelvic disproportion in women attempting vaginal birth after previous cesarean delivery Gary R. Thurnau, MD, David H. Scates, MD, and Mark A. Morgan, MD Oklahoma City, Oklahoma In 1986 the fetal-pelvic index was introduced as an accurate method of prospectively identifying the presence or absence of fetal-pelvic disproportion. The concept of the fetal-pelvic index is one in which the fetal head and abdominal circumferences (ultrasonographic mensuration) are compared with the respective maternal pelvic inlet and midpelvic circumferences (x-ray pelvimetry). The purpose of this study is to evaluate the efficacy of this index as a predictor of fetal-pelvic disproportion in gravid women attempting vaginal birth after previous cesarean delivery. Findings of the fetal-pelvic index are compared with those of two other means of identifying fetal-pelvic disproportion (Colcher-Sussman x-ray pelvimetry and ultrasonographically derived estimated fetal weight of ~4000 gm). Of the 65 prospective study participants, 18 (28%) failed to progress in labor, and a repeat cesarean delivery was required. Forty-seven of the 52 patients with a negative fetal-pelvic index value were delivered vaginally (negative predictability = 0.90). Of the five with false-negative fetal-pelvic index values, four fetuses persisted in an oCcipitoposterior position and failed to progress in labor. All 13 patients with a positive fetal-pelvic index value failed to progress in labor and required a cesarean delivery (positive predictability = 1.00). In contrast, when used alone, neither x-ray pelvimetry nor ultrasonography-determined estimated fetal weight of ~4000 gm provided accurate identification of fetal-pelvic disproportion. (AM J OBSTET GVNECOL 1991 ;165:353-8.)

Key words: Fetal-pelvic disproportion, fetal-pelvic index, vaginal birth after previous cesarean delivery

Since 1970 the dramatic increase in the rate of cesarean section has made it the most frequently performed major operation in the United States.'-' furthermore, elective repeat cesarean has become the most common indication for cesarean delivery.4.5 Although the safety of vaginal birth after previous cesarean delivery has been well documented,6-10 elective repeat cesarean remains the rule in most labor and delivery units. Flamm et al. 11 reported on the frequency of vaginal birth after previous cesarean delivery and found it to be related to the indication for the prior cesarean delivery. In their multicenter study of 1776 labor trials, women with failure to progress or cephalopelvic disproportion as the documented indication for cesarean

From the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, University of Oklahoma College of Medicine. Received for publication June 17, 1990; revised January 22, 1991; accepted January 28, 1991. Reprint requests: Gary R. Thurnau, MD, Depart. of Obstetrics and Gynecology, University of Oklahoma College of Medicine, P.O. Box 26901, Oklahoma City, OK 73190.

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delivery had the lowest success rate (65%) for subsequent vaginal birth whereas those women with breech presentation as the indication had the highest success rate (88%). Although approximately 75% of women attempting vaginal birth after cesarean delivery reportedly are successfuV 1 a method of prospectively identifying fetal-pelvic disproportion could prevent prolonged unsuccessful labor trials in the majority of women destined to fail the attempt. In 1986 the fetal-pelvic index was introduced as an accurate method of prospectively identifying the presence or absence of fetal-pelvic disproportion. 12 The concept of the fetal-pelvic index is one in which the fetal head and abdominal circumferences (passenger) are compared with the respective maternal pelvic inlet and mid pelvic circumferences (passage). The purpose of this study is to evaluate the accuracy of the fetalpelvic index as a predictor of fetal-pelvic disproportion and delivery outcome in gravid women attempting vaginal birth after previous cesarean delivery. Results of the fetal-pelvic index data are compared with those of two other means of identifying fetal-pelvic disproportion (Colcher-Sussman x-ray pelvimetry l3 and ultra-

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Table I. Results of measurement data* Ultrasonography (cm) Birth weight Delivery outcome

No.

(gm)

Fetal head circumference

Cesarean delivery Vaginal delivery p valuet

18 47

3486 ± 393 3325 ± 514 NS

33.0 ± 1.6 32.8 ± 1.2 NS

X-ray pelvimetry (cm)

Fetal abdomen circumference

Maternal pelvic inlet circumference

Maternal midpelvis circumference

35.1 ± 2.1 33.7 ± 1.9