Objective
We sought to compare labor patterns by body mass index (BMI).
Study Design
A total of 118,978 gravidas with a singleton term cephalic gestation were studied. Repeated-measures analysis constructed mean labor curves by parity and BMI categories for those who reached 10 cm. Interval-censored regression analysis determined median traverse times, adjusting for covariates in vaginal deliveries and intrapartum cesareans.
Results
In the labor curves, the time difference to reach 10 cm was 1.2 hours from the lowest to highest BMI category for nulliparas. Multiparas entered active phase by 6 cm, but reaching this point took longer for BMI ≥40.0 (3.4 hours) compared to BMI <25.0 (2.4 hours). Progression by centimeter ( P < .001 for nulliparas) and from 4-10 cm ( P < .001 for nulliparas and multiparas) increased as BMI increased. Second stage length, with and without an epidural, was similar among BMI categories for nulliparas ( P > .05) but decreased as BMI increased for multiparas ( P < .001).
Conclusion
Labor proceeds more slowly as BMI increases, suggesting that labor management be altered to allow longer time for these differences.
Cesarean deliveries (CD) have reached an all-time high in the United States–estimated at 32.9% in 2009. Explanations for these findings include delayed childbearing, more multiple gestations, fewer attempts at vaginal birth after CD, and increasing maternal body mass. In parallel with this effect is the obesity epidemic. In 2007 through 2008, the prevalence of obesity among adults was 33.8%. There is a linear rise in CD as maternal body mass index (BMI) increases. The explanation for this finding is unknown, but several theories have been proposed, including greater fetal size, soft tissue obstruction to labor, poor uterine contractility, more frequent inductions as a result of pregnancy complications, or caregiver biases.
The Friedman curve is a well-known figure to all clinicians who participate in the management of labor and delivery. However, the evaluation of labor progression is largely based on studies from the 1950s that described patients from a small homogeneous population. Subsequent studies have suggested that this curve may not apply to current obstetrical care. The Consortium on Safe Labor is a multicentered, retrospective study of electronic obstetrical databases whereby labor progression in patients with contemporary obstetrical characteristics (ie, increasing maternal age and BMI) and practices (ie, a greater proportion of patients with inductions and epidurals) was studied. The primary investigation from this database determined that cervical dilation progresses more slowly than previously described, especially at cervical dilations from 4-6 cm. Few studies have addressed the specific impact of BMI on labor progression. The purpose of the current study was to characterize labor progression in gravidas with respect to their BMI at labor admission. If differences in labor patterns based on maternal BMI are found, this may help optimize labor management and ultimately impact the CD rate.
Materials and Methods
The Consortium on Safe Labor abstracted detailed labor and delivery information from deliveries occurring between 2002 and 2008 from electronic medical records in 12 clinical centers (with 19 hospitals) across 9 American College of Obstetricians and Gynecologists US districts. Detailed descriptions of the study are provided elsewhere. Data on labor progression (repeated, time-stamped cervical dilation, station, and effacement) were collected from the electronic labor database. Labor management protocols, including the timing and frequency of cervical examinations performed during labor, varied across institutions. The institutional review boards of all participating institutions approved the study.
There were a total of 228,668 deliveries (87% of which occurred during 2005 through 2007) in the database. For the current study, deliveries were excluded based on the following criteria: unknown BMI on admission (n = 48,005); multiple gestation (n = 5059); not term (outside gestational age of 37 0/7 –41 6/7 weeks; n = 31,149); stillbirth (n = 2046); breech fetal presentation (n = 7764); prior CD (n = 33,020); and no documented trial of labor (defined as at least 2 cervical examinations in the obstetrical database; n = 36,407). The number of deliveries remaining, noting that some deliveries met >1 criterion, was 126,257. To avoid intraperson correlation, we selected the first delivery from each participant in the study, leaving 118,978 deliveries for analysis ( Figure 1 ) .
Women were then grouped into either nulliparas or multiparas. BMI categories were determined by World Health Organization criteria (underweight or normal weight, <25.0 kg/m 2 ; overweight, 25.0-29.9 kg/m 2 ; obese class I, 30.0-34.9 kg/m 2 ; obese class II, 35.0-39.9 kg/m 2 ; and obese class III, ≥40 kg/m 2 ). Other descriptive variables included maternal age, height, race, gestational age, diabetes (pregestational and gestational combined), cervical dilation and effacement at admission, augmentation, induction, operative vaginal delivery, epidural anesthesia, CD, and birthweight. A cesarean for abnormal labor was one that was documented as “failure to progress” or “cephalopelvic disproportion” in the database. Pearson χ 2 tests were used to compare the association between categorical variables and BMI category, and analysis of variance was used to statistically compare the association between continuous variables and BMI category.
A repeated-measures analysis with an eighth-degree polynomial model was used to construct mean labor curves by parity using cervical dilation in centimeters with Proc Mixed (SAS software, version 9.2; SAS Institute Inc, Cary, NC). In this analysis, the starting point was set at the first time when the dilation reached 10 cm (time = 0) and the time was calculated backward (eg, 60 minutes prior to complete dilation = –60 minutes). After the labor curve models had been computed, the x-axis (time) was reverted to a positive value (eg, instead of –12 to 0 hours, it was transformed to 0-12 hours). The labor curves included gravidas who reached 10 cm, including those who may have had a cesarean in the second stage. Because a large proportion of women were not admitted to labor and delivery until cervical dilation was ≥4 cm, we started our labor curves at 4 cm. Gravidas dilated >4 cm on admission still contributed data to the labor curve, as long as there were at least 2 cervical examinations in the database. S-Plus software, version 6.1 (Insightful, Seattle, WA), was used to create the labor curve graphs.
To estimate duration of labor, we used interval-censored regression to determine the distribution of times for progression from 1 cm of dilation to the next (called “traverse time”), with the assumption that the labor data were log-normally distributed. Although this analysis was also restricted to laboring gravidas, it included gravidas who ultimately had either a vaginal delivery or an intrapartum cesarean (including cesareans done in the first and second stages). The median and 95th percentiles were calculated for the first stage of labor, and the χ 2 test was used to test the significance of the BMI groups. The median and 95th percentiles for the second stage were derived from Proc Univariate (SAS software, version 9.2; SAS Institute). Tests of a continuous BMI covariate were used in GLM (SAS software, version 9.2; SAS Institute) to compare the trends in labor time as BMI increased. We compared the traverse times in the first stage of labor and the duration of the second stage, adjusting for the combined observed frequencies of maternal age, height, race, gestational age, diabetes, induction, augmentation, epidural (first stage only), operative vaginal delivery (second stage only), and birthweight using Proc Lifereg (SAS software, version 9.2; SAS Institute). Trends in the traverse times with BMI were obtained as tests of a continuous BMI covariate in the Lifereg model. A P value < .05 was considered statistically significant.
Results
The characteristics of the population according to parity and BMI category are presented in Tables 1 and 2 . The mean admission BMI was 30.5 kg/m 2 , and 7.3% of gravidas had a BMI ≥40 kg/m 2 . Nulliparas represented 48% of the population, and 22% had a CD. As BMI increased, so did the occurrence of black race, diabetes, number of cervical examinations, inductions, CD, and CD for labor abnormalities, while the opposite was found for white race, admission cervical dilation and effacement, and operative vaginal delivery in nulliparas ( P < .001) and multiparas ( P < .05). Nulliparas had decreased age, cervical dilation at admission, and diabetes compared to multiparas but increased gestational age, operative vaginal deliveries, epidurals, and CD compared to multiparas across all BMI categories.
| BMI categories (kg/m 2 ) at admission | |||||||
|---|---|---|---|---|---|---|---|
| Characteristic | <25.0 | 25.0-29.9 | 30.0-34.9 | 35.0-39.9 | ≥40.0 | Total | P value a |
| Total | 9097 | 23,588 | 14,719 | 6113 | 3945 | 57,462 | |
| Maternal age, y, mean (SD) | 24.7 (6.1) | 25.4 (6.1) | 25.0 (5.9) | 24.7 (5.7) | 24.6 (5.5) | 25.1 (6.0) | < .0001 |
| Maternal height, m, mean (SD) | 1.65 (0.07) | 1.63 (0.07) | 1.63 (0.07) | 1.63 (0.08) | 1.62 (0.09) | 1.63 (0.07) | < .0001 |
| Maternal weight (kg) at labor admission, mean (SD) | 63.5 (6.3) | 73.6 (7.2) | 85.7 (8.3) | 98.7 (10.0) | 119.1 (17.5) | 80.9 (17.0) | < .0001 |
| White race, n (%) | 4740 (52.1) | 12,624 (53.5) | 7295 (49.6) | 2874 (47.0) | 1710 (43.4) | 29,243 (50.9) | < .0001 |
| Black race, n (%) | 1603 (17.6) | 3899 (16.5) | 3231 (22.0) | 1646 (26.9) | 1389 (35.2) | 11,768 (20.5) | |
| Hispanic race, n (%) | 1418 (15.6) | 4045 (17.2) | 2771 (18.8) | 1120 (18.3) | 586 (14.9) | 9940 (17.3) | |
| Gestational age, wk, mean (SD) | 39.2 (1.1) | 39.4 (1.1) | 39.5 (1.1) | 39.4 (1.2) | 39.4 (1.2) | 39.4 (1.1) | < .0001 |
| Diabetes, n (%) | 188 (2.1) | 625 (2.7) | 550 (3.7) | 365 (6.0) | 352 (8.9) | 2080 (3.6) | < .0001 |
| Dilation at admission, cm, mean (SD) | 3.2 (2.1) | 2.9 (2.1) | 2.7 (2.0) | 2.6 (2.0) | 2.3 (1.9) | 2.9 (2.1) | < .0001 |
| Effacement at admission, n (%), mean (SD) | 79.8 (22.2) | 76.6 (22.8) | 73.0 (24.4) | 69.6 (26.0) | 65.0 (27.9) | 74.7 (24.0) | < .0001 |
| No. of cervical examinations, mean (SD) | 7.3 (3.1) | 7.7 (3.3) | 8.1 (3.5) | 8.3 (3.8) | 8.4 (3.9) | 7.8 (3.4) | < .0001 |
| Induction, n (%) | 3634 (40.0) | 10,296 (43.7) | 7114 (48.3) | 3334 (54.5) | 2415 (61.2) | 26,793 (46.6) | < .0001 |
| Augmentation, n (%) | 3421 (37.6) | 9137 (38.7) | 5798 (39.4) | 2253 (36.9) | 1343 (34.0) | 21,952 (38.2) | < .0001 |
| Epidural analgesia, n (%) | 7369 (81.0) | 19,528 (82.8) | 12,273 (83.4) | 5049 (82.6) | 3205 (81.2) | 47,424 (82.5) | < .0001 |
| Operative vaginal delivery, n (%) | 1112 (12.2) | 2721 (11.5) | 1453 (9.9) | 469 (7.7) | 214 (5.4) | 5969 (10.4) | < .0001 |
| Cesarean delivery, n (%) | 1063 (11.7) | 4348 (18.4) | 3846 (26.1) | 2110 (34.5) | 1769 (44.8) | 13,136 (22.9) | < .0001 |
| Cesarean performed for labor abnormality, n (%) | 624 (58.7) | 2647 (60.9) | 2463 (64.0) | 1329 (63.0) | 1136 (64.2) | 8199 (62.4) | .0014 |
| Birthweight, g, mean (SD) | 3166 (408) | 3305 (420) | 3391 (437) | 3414 (460) | 3424 (471) | 3325 (438) | < .0001 |
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