Objective
The purpose of this study was to estimate the usefulness of the fetal-pelvic index (FPI) in the prediction of cesarean delivery among nulliparous and women who undergo a trial of labor after cesarean delivery (TOLAC).
Study Design
This prospective cohort study included subjects at 2 hospitals from the University of Pennsylvania Health system. The study sample included nulliparous women and women who attempted TOLAC, with nonanomalous pregnancies at ≥37 weeks of gestation in vertex presentation (n = 221 and 207, respectively). FPI score was calculated with the ultrasound-based fetal biometric measures that were performed within 2 weeks of delivery and x-ray pelvimetry that was performed within 48 hours of delivery. Multivariable logistic regression was used to develop a clinical predictive index for cesarean delivery, which included FPI and clinical factors, in nulliparous women or women who attempted TOLAC. The prediction models were tested for accuracy with the area under the receiver operating characteristics curve.
Results
Higher FPI scores were associated with greater odds of cesarean delivery. A unit increase in FPI score increased the odds of cesarean delivery by 15% (adjusted odds ratio, 1.15; 95% confidence interval, 1.09–1.21) for nulliparous women and 15% for women who attempted TOLAC (adjusted odds ratio, 1.15; 95% confidence interval, 1.10–1.20) after adjustment for maternal age, race, medical risk factors, and labor method. Among nulliparous women, the receiver operating characteristics analysis estimated an area under the curve of 0.88, with positive and negative predictive values of 76% and 87%, respectively. Similar findings were observed in the subgroup of women who attempted TOLAC.
Conclusion
The FPI when combined with clinical risk factors can identify accurately women who are at a high risk for cesarean delivery.
Although cesarean delivery, compared with vaginal birth, is associated with increased maternal and neonatal morbidity and mortality rates, previous research has indicated that cesarean delivery performed after a patient has been in labor is associated with highest rates of morbidity and mortality when compared with vaginal delivery or elective cesarean delivery The ability to identify those patients who are at highest risk for cesarean delivery and to avoid a prolonged course of labor and increased complications would be clinically valuable.
Prediction models that were developed for cesarean delivery have incorporated variables such as the Bishop scores and other antepartum and intrapartum factors. Some of these models have shown limited accuracy ; others are too complicated to be applicable in routine clinical practice. In addition, the use of intrapartum factors in such prediction studies assumes that a patient is in labor when the scoring system is applied and that counseling about likely outcomes cannot be made until labor occurs. From the perspectives of both the patient and obstetrician, this is not ideal. Unfortunately, previous research that used antepartum factors alone in a predictive index (without intrapartum factors) had yielded inferior test characteristics than when both intrapartum and antepartum factors are included.
One of the most common reasons for cesarean delivery is fetal-pelvic disproportion or an inadequately sized maternal pelvis relative to fetal biometry. In relatively small studies, the fetal-pelvic index (FPI) has been shown to have good sensitivity and specificity for the identification of the presence of fetal-pelvic disproportion prospectively. The FPI provides a method of comparing the “size” of the fetus at term with ultrasound scanning with the size of the maternal pelvis at x-ray pelvimetry. The clinical principle behind the FPI is simply that, if the estimated size of the fetus by conventional ultrasound scan is greater than the size of the maternal pelvis, then the likelihood of a cesarean delivery because of this disproportion is high. To date, the implementation of FPI has been limited. The objective of this study was to estimate whether FPI, combined with clinical risk factors, could be used to predict cesarean delivery and to develop a clinical prediction tool for the risk of cesarean delivery in nulliparous women or women who attempted a trial of labor after cesarean delivery (TOLAC). We further sought to develop a predictive nomograph; a graphic illustration of the independent contribution of multiple factors to help in the prediction for cesarean delivery.
Materials and Methods
We performed a prospective cohort study from 2001-2006 to estimate the utility of the FPI in the prediction of the likelihood of a cesarean delivery that is performed for cephalopelvic disproportion. Women with singleton pregnancy and cephalic fetal presentation at 36-42 weeks’ gestation were recruited at 2 urban hospitals. Subjects with multiple gestations and major fetal malformations were excluded from the study. The study was focused initially on women who were attempting TOLAC but was expanded to nulliparous subjects because the rate of TOLAC declined during the years of the study. The study sample consisted of 221 nulliparous women and 207 women who were attempting TOLAC with only 1 previous cesarean delivery. The study was approved by the institutional review board of all participating institutions.
Women at participating sites were screened for eligibility by trained research nurses, and eligible women were offered enrollment into the study. Patients who agreed to participate underwent an in-person interview and had detailed abstraction of their prenatal record. Enrolled subjects had 3 ultrasound examinations performed to measure fetal biometry weekly beginning at 36 weeks’ gestation by obstetric research nurses who were trained by a sonologist beginning at 36 weeks’ gestation. The main measures of interest were the fetal head circumference and abdominal circumference. The ultrasound examination that was performed closest to delivery was used for our calculations of the FPI. Subjects’ intrapartum care providers were blinded to the results of the ultrasound examination, unless unexpected fetal malpresentation (ie, breech or transverse lie) was detected. The intrapartum care was at the discretion of the attending physicians and included decisions about labor induction/augmentation, operative vaginal delivery, and the need for cesarean delivery.
The other components of the FPI are measures of the maternal pelvis. Because of concerns regarding radiation exposure during pregnancy, x-ray pelvimetry was performed after the delivery and measured within 48 hours of delivery as a surrogate for predelivery measures. Importantly, in a previous study, we had validated a high concordance between predelivery and postdelivery x-ray pelvimetry. X-ray pelvimetry consisted of the anteroposterior and transverse diameters of the inlet and mid pelvis with the Colcher-Sussman method. The reading of the x-ray pelvimetry was performed by the principal investigator. Blinding of pelvimetry reading was attempted, although in a small number of cases (<10%) surgical staples were used for skin closure. In these cases, a second reader reread these films and calculated pelvic diameters. There was a high degree of agreement between these 2 readings (intraclass correlation coefficient, 0.90).
The combination of predelivery fetal ultrasound examination and postpartum x-ray pelvimetry was used for the calculation of the “FPI”, as described by Morgan et al. Ultrasonographic measurements of fetal head and abdominal circumferences were compared with respective maternal pelvic inlet and mid pelvic circumferences by x-ray pelvimetry. Specifically, individually measured anteroposterior and transverse diameters of the fetal cranium, fetal abdomen, maternal pelvic inlet, and maternal mid pelvis are used to compute respective circumferences of the fetal head (HC), fetal abdomen (AC), maternal pelvic inlet (IC), and maternal midpelvis (MC). On the basis of 4 circumference differences between the fetus and maternal pelvis (HC-IC, HC-MC, AC-IC, AC-MC), a value was derived from the sum of the 2 most positive circumference differences by mathematic computation. The FPI was classified as either positive or negative. A positive index value identifies a fetus that is larger than the maternal pelvis, and a negative index value is smaller than the maternal pelvis. The primary outcome of the study was cesarean delivery for any indication. We also collected detailed information on other potential predictors of cesarean delivery from an in-person interview and a review of the medical record and the records of previous deliveries, when applicable.
Statistical analysis
Nulliparous women and women who underwent TOLAC were analyzed separately. Univariable analysis was conducted to summarize sample characteristics; the distribution of the FPI scores in our study sample was examined. The odds of cesarean delivery were modeled with the use of multivariable logistic regression analysis. Adjusted odds ratios and the corresponding 95% confidence interval (CI) were estimated. In our multivariable prediction model FPI, we included maternal age, maternal race, maternal stress during pregnancy, labor method (ie, spontaneous, augmented, or induced), income, and a binary composite indicator of maternal comorbidity (ie, preexisting diabetes mellitus, heart disease, hypertension, autoimmune system disorder, kidney disease, and neurologic disorders). The selection of covariates was based on their clinical significance and association with cesarean delivery. Both univariable and multivariable analysis were performed only with subjects with complete data for the covariates that were included in the multivariable models. We used receiver operator characteristics (ROC) curve analysis to estimate the accuracy of the logistic regression model in predicting the likelihood of cesarean delivery. The probability value for all hypothesis tests was 2-sided. Statistical analyses described earlier were completed with STATA software package (version 10, Special Edition; STATA Corp LP, College Station, TX).
A graphic nomogram was also generated from the logistic regression model. The model coefficients were used to create the nomogram that allowed for a visualization of the magnitude of the associations between each predictor and cesarean delivery. The nomogram was constructed with the nomogram function in the Design package of Frank Harrell (Department of Biostatistics, Vanderbilt University, f.harrell@vanderbilt.edu ) with the R language. The nomogram is based on converting each regression coefficient (effect) in the multivariable logistic regression model to a 0-100 scale that is proportional to the log odds. To cross-validate our predictive models, we divided the original data set into a training set and a test set separately for nulliparous and women who attempted TOLAC.
Results
Of the 428 women who were included in the analysis, 133 women (31%) had a cesarean delivery. Similar to other published findings, the rate of cesarean delivery was higher among women who underwent TOLAC (36%) than among nulliparous women (26%). Importantly, 25% of our TOLAC subjects had a previous vaginal delivery. Table 1 gives a summary of the sample characteristics. For both nulliparous women and those who underwent TOLAC, our study sample consisted of mostly African American women who had at least 1 medical risk factor, medium stress level during pregnancy, augmented labor, and a lower income level. Most of our study subjects had a negative FPI value that indicated a favorable maternal pelvis relative to fetal biometry. The distribution of FPI scores for nulliparous women and those who underwent TOLAC are presented in Figures 1 and 2 .
| Characteristics | Nulliparous women (n = 221) | Women who attempted a trial of labor after a cesarean delivery (n = 207) a |
|---|---|---|
| Fetal pelvic index score | ||
| Mean ± SD | –10.77 ± 15.69 | –13.19 ± 36.22 |
| Positive fetal pelvic index, n (%) | 47 (21.3) | 47 (22.7) |
| Negative fetal pelvic index, n (%) | 174 (78.7) | 160 (77.3) |
| Maternal age, y b | 21.6 ± 4.73 | 26.4 ± 6.3 |
| Maternal race: African American, n (%) | 149 (69.6) | 107 (53.5) |
| Infant birthweight, g b | 3330 ± 444 | 3426 ± 467 |
| Gestational age, wk b | 39.6 ± 1.24 | 39.5 ± 1.10 |
| Medical risk factor composite, n (%) c | 88 (42.9) | 85 (42.1) |
| Labor method, n (%) | ||
| Spontaneous | 60 (27.9) | 62 (30.4) |
| Augmented | 116 (54.0) | 91 (44.6) |
| Induced | 39 (18.1) | 51 (25.0) |
| Income, n (%) | ||
| ≤$10,000 | 116 (58.0) | 51 (25.9) |
| $10,001-30,000 | 51 (25.5) | 66 (33.5) |
| $30,001-50,000 | 11 (5.5) | 21 (10.7) |
| >$50,000 | 22 (11.0) | 59 (29.9) |
a Subjects who had attempted a trial of labor in the index pregnancy after a previous cesarean delivery
c Includes antepartum diabetes mellitus, hypertension, heart disease, autoimmune system disorder, kidney disease, and neurologic disorders.
Our multivariable analysis revealed that FPI scores are associated positively with the odds of cesarean delivery. Specifically, after being controlled for covariates, each unit increase in FPI score was associated with a 15% increase in the odds of cesarean delivery in both nulliparous and TOLAC subjects ( Table 2 ). Binary logistic regression models were developed to generate ROC curves of the likelihood of cesarean delivery after an attempt at labor as a function of FPI scores and other maternal and fetal predictors. In a model with only FPI as a predictor, the areas under the curve (AUC) were 85% and 83% for nulliparous and women who attempted TOLAC, respectively (data not shown). The addition of maternal and fetal characteristics improved the predictability of the model modestly. After adjustment for maternal age, maternal race, labor method, and a binary composite indicator of medical risk factors, AUC were 88% (95% CI, 0.83–0.93) for nulliparous women and 89% for women who attempted TOLAC ( Figures 3 and 4 ). Among the nulliparous women, the sensitivity, specificity, and positive and negative predictive values for the multivariable model were 60% (95% CI, 0.53–0.67), 93% (95% CI, 0.89–0.97), 76% (95% CI, 0.70–0.82), and 87% (95% CI, 0.82–0.92), respectively. Among the women who attempted TOLAC, the sensitivity, specificity, positive predictive value, and negative predictive value for the multivariable model were 65% (95% CI, 0.58–0.72), 90% (95% CI, 0.86–0.94), 78% (95% CI, 0.72–0.84), and 82% (95% CI, 0.77–0.88), respectively.
| Characteristics | Nulliparous women (n = 221) | Women who attempted a trial of labor after a cesarean delivery (n = 207) | ||
|---|---|---|---|---|
| Crude OR (95% CI) | Adjusted OR (95% CI) | Crude OR (95% CI) | Adjusted OR (95% CI) | |
| Fetal pelvic index score | 1.14 (1.08–1.20) | 1.15 (1.09–1.21) | 1.16 (1.11–1.21) | 1.15 (1.10–1.20) |
| Maternal age | 1.08 (1.01–1.16) | 1.13 (1.01–1.26) | 1.04 (0.99–1.09) | 1.10 (1.02–1.19) |
| Maternal race | ||||
| African American | 1.28 (0.59–2.75) | 1.08 (0.36–3.19) | 1.91 (1.02–3.57) | 2.48 (0.91–6.76) |
| Medical risk factor composite a | 0.64 (0.32–1.30) | 0.76 (0.32–1.79) | 1.21 (0.65–2.26) | 1.22 (0.53–2.79) |
| Labor method | ||||
| Spontaneous | Reference | Reference | Reference | Reference |
| Augmented | 3.23 (1.15–9.08) | 3.38 (0.98–11.68) | 1.35 (0.64–2.83) | 1.30 (0.49–3.43) |
| Induced | 7.03 (2.10–23.50) | 5.00 (1.17–21.47) | 1.78 (0.76–4.15) | 2.52 (0.82–7.79) |
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
