The Role of Oxytocin in Primary Cesarean Birth Among Low-Risk Women

Abstract

Introduction:

To examine whether there is a threshold of oxytocin exposure at which the risk for primary cesarean increases among women who are nulliparous with a term, singleton, vertex fetus (NTSV) and how oxytocin interacts with other risk factors to contribute to this outcome.

Methods:

This was a secondary analysis of the Consortium on Safe Labor data set that used a retrospective cohort study design. Women who met the criteria for NTSV who were not admitted for a prelabor cesarean and for whom oxytocin data were available, were included in the sample. Robust logistic regression was used to examine the association of oxytocin exposure with primary cesarean birth, while controlling for demographic and clinical risk factors and clustering by provider.

Results:

The sample comprised 17,331 women who were exposed to oxytocin during labor. The women were predominantly white non-Hispanic (59.2%) with an average (SD) gestational age of 39.4 (1.1) weeks and an 18.5% primary cesarean rate. Exposure to greater than 11,400-milliunits (mU) of oxytocin resulted in 1.6 times increased odds of primary cesarean birth compared with less than 11,400 mU (95% CI 1.01-2.6).

Discussion:

Exposure to greater than 11,400 mU of oxytocin in labor was associated with an increased odds of primary cesarean birth in NTSV women.

INTRODUCTION

One-third of births in the United States occur via cesarean. Reducing the primary cesarean birth rate, especially among women who are nulliparous with a term, singleton, vertex fetus (NTSV), is a focus of quality and safety initiatives.^1-3 Although maternal-, health care provider-, and system-level factors influence primary cesarean birth rates, not all of them are amenable to intrapartum intervention.^4-12 Oxytocin, a medication widely used in US intrapartum care to induce and augment labor, is associated with an increased risk for cesarean birth.^13,14 This association may be mitigated by following guidelines for use, including requiring a medical indication, or turning the medication off when a woman is in active labor.^4,15,16

Little is known about the threshold dose at which oxytocin is associated with an increased risk for primary cesarean birth. Although there is evidence in the literature that oxytocin interacts with other risk factors, such as maternal body mass index (BMI), it is not clear how these interactions influence the likelihood of a primary cesarean birth. To address these gaps, this study explored the association between the total dose of oxytocin exposure in labor and the incidence of primary cesarean in women who were NTSV to identify potential risk thresholds and examined how exposure to oxytocin may interact with other risk factors in predicting the outcome of primary cesarean birth. Low risk in this study has been specifically defined as NTSV.

METHODS

A secondary data analysis of the Consortium on Safe Labor (CSL) data set was conducted using a retrospective cohort design. The CSL is a publicly available database initially developed to examine patterns in labor progression and the etiology of cesarean births in the United States. The data are housed at the Eunice Kennedy Shriver National Institute of Child Health and Human Development’s Data and Specimen Hub. Data for the CSL were collected from 12 sites (19 hospitals), representing the American Congress of Obstetricians and Gynecologists’ 9 districts. For the parent study, site investigators extracted data from the electronic health records. Hospital and provider characteristics were collected from surveys of local investigators. Data were transferred from the sites to the Data Coordinating Center, where data were checked, cleaned, and recoded. Four key outcome measures (cesarean for nonreassuring fetal heart tone, asphyxia, shoulder dystocia, and neonatal intensive care unit admission for respiratory conditions) were validated by comparing the results of the electronic health record data extraction with the results of a manual chart extraction There was a high degree of concordance between the electronic and manual chart extractions. The lowest rate of concordance was 93.3%; nonreassuring fetal heart tones as an indication for cesarean birth varied between the electronically and manually extracted data in 3.5% of charts.¹⁷ The database includes 228,668 births that occurred between 2002 and 2008, with 9.5% of women contributing more than one birth. Patient information was deidentified before being sent to the Data Coordinating Center.

For this study, primary cesarean birth was the outcome variable of interest. To generate the sample, sites were included if oxytocin dosing information was collected. Inclusion criteria were (1) gestational age greater than or equal to 37 weeks and less than 42 weeks, (2) parity of 0, (3) singleton fetus, (4) vertex presentation, and (5) exposure to oxytocin during labor. Sample exclusion criteria for women included (1) no exposure to oxytocin intrapartum, (2) incomplete oxytocin dose information (ie, missing oxytocin dose or duration), and (3) admission for a prelabor cesarean birth.

The primary independent variable was total dose of oxytocin in milliunits (mU). Total dose, which has been used previously as a measure for oxytocin exposure, was calculated for each woman by summing each oxytocin rate (mU per minute) by the duration of time (minute) the woman received that rate.^18,19 As a continuous variable, oxytocin exposure spanned a wide, right-skewed range (2 mU to 104,374 mU); therefore, a categorical variable for oxytocin exposure was created. Categories of 300-mU increments were chosen to balance granularity with parsimony Because categories of 300-mU increments resulted in almost 350 categories of oxytocin exposure; these were grouped into 5 categories (2-2400 mU; 2401-11,400 mU; 11,401 mU-14,700 mU; 14,701-21,000; and 21,001-104,374 mU) based on points in oxytocin exposure where the rate of cesarean birth changed in a statistically significant manner. Of the 5 categories, the first 2 and the last 3 were not statistically significantly different from each other with regard to the outcome of cesarean birth and were combined to form 2 categories of oxytocin exposure. The first total oxytocin dose ranged from 2 mU to 11,400 mU, and the second ranged from 11,401 mU to 104,374 mU.

Other independent variables, including maternal race and ethnicity, age, and gestational age, were collected from the labor and delivery admission form or the reproductive and health care history forms. Race and ethnicity included white, non-Hispanic; Black, non-Hispanic; Hispanic; Asian or Pacific-Islander; multiracial; other; and unknown. Because of the paucity of women in the multiracial category (n = 6), this category was combined with other and unknown. Maternal age (<20, 20-34, ≥35 years), gestational age (37 0/7-38 6/7 weeks, 39 0/7-40 6/7 weeks, ≥41 0/7 weeks), and BMI (<24.9, normal; 25-29.9, overweight; ≥30, obese) were categorized as noted.²⁰ The BMI variable was missing for 18% of women, so a proxy BMI variable was created based on maternal height and admission weight (2.6% missing). Chronic and gestational diabetes, gestational hypertension, superimposed preeclampsia, severe and moderate preeclampsia, and eclampsia were all derived from the participants’ charts. Chronic hypertension was based on an International Classification of Diseases, Ninth Revision code and chart data, whereas administration of magnesium sulfate was collected from the labor and delivery summary. All of the maternal condition variables were binary. Hospital type described whether the hospital was a university teaching, community teaching, or community nonteaching hospital.

No information was missing on mode of birth, maternal race, gestational age, or hospital type. Race was listed as unknown for 812 women, and these women were included in the regression by using a dummy variable. The variables any hypertension and any diabetes were coded 1 if the woman had any hypertensive or diabetic disorder and 0 if she did not have the disorder or the information was missing. Two of the women in the study sample were missing a value for maternal age and were dropped from the regression model. Identification numbers were missing for 11 attending providers, and these, too, were not used in the regression. Missing proxy BMI was given a dummy variable, and these women were kept in the regression analysis. We compared women with and without complete oxytocin dosing information, as only women who had complete dose data were included in this analytic sample.

The demographic profile of the study sample was summarized with mean and SD, frequency and percentage, or median and interquartile range, depending on the distribution of the variable. The goal of the analytic sequence was to determine the association between oxytocin exposure, as characterized by the 2-category variable, and primary cesarean birth in women who were NTSV while controlling for other confounding variable at the person, health care provider, and system level.

A robust logistic regression model with primary cesarean birth as the outcome was performed, adjusted for oxytocin exposure, maternal age, race, BMI, interaction of BMI and oxytocin, gestational age, any hypertensive disorders, any diabetic disorders, and hospital type. To control for differences in health care provider practice, data from individual women were clustered by the provider who attended their birth. An interaction term for BMI and oxytocin was included in the regression because the literature indicates that the effect of synthetic oxytocin varies across BMI categories.^20-24 The interaction term allows us to understand the effect of receiving greater or less than 11,400 mU of oxytocin on the odds of having a cesarean birth at different BMI categories. Antenatal large for gestational age status, which was collected from the participant’s prenatal history, was initially included and later pruned from the model because it was collinear with diabetes. A sensitivity analysis was conducted, stratifying the regression by induction of labor, which was treated as was a binary variable.

According to the a priori power analysis, assuming the frequency of oxytocin exposure was 50%²⁵ and the rate of primary cesarean was 26.9%,²⁶ 3400 births would be required to have 80% power to detect an odds ratio of 1.3 for oxytocin as a predictor of cesarean births at P level .05. In the literature, the adjusted odds ratios (aORs) for having the outcome of a cesarean, given an induction of labor (not, specifically, oxytocin exposure), are commonly greater than 1.3.²⁷ The power analysis was conducted on PASS 14.0.²⁸ The Johns Hopkins University institutional review board declared the current analysis of oxytocin and primary cesarean birth using the CSL data exempt (IRB00150587). STATA²⁹ was used for all analyses.

RESULTS

Of 64,447 women with available oxytocin data, 38,533 were excluded for not being nulliparous or not having a vertex, term, singleton pregnancy; 4810 were excluded with prelabor cesarean births; 2496 were excluded for missing oxytocin data; 1217 were excluded with oxytocin values that indicated postpartum exposure only, no exposure (ie, a total dose of 0), or greater than 80 hours of oxytocin administration; and 58 were excluded for being postterm gestational age. The inclusion and exclusion criteria yielded an analytic sample of 17,331 women. The sample came from 5 sites: 4 university teaching hospitals and one site comprising 5 community hospitals (3 teaching and 2 nonteaching). Births were attended by 527 different providers. White non-Hispanic women made up 59.2% of the sample, with Black non-Hispanic women making up 17.3% and Hispanic women making up 14.9% (Table 1). The women in this sample has an average gestational age of 39.4 (SD, 1.1) weeks, with a mean BMI on admission of 30.4 (SD, 5.9).

Table 1.

Demographic Characteristics of 17,331 Women Who Are Nulliparous with a Term, Singleton, Vertex Fetus Exposed to Oxytocin in Labor

Variables	Total Sample n (%) n = 17,331	Oxytocin ≤11,400 mU n (%) n = 14,905	Oxytocin >11,400 mU n (%) n = 2426	P Value
Age, y				0.3
≤19	3277 (18.9)	2819 (18.99)	458 (18.4)
20-34	13,354 (77.1)	11,437 (77.05)	1917 (77.1)
≥35	700 (4)	587 (3.95)	113 (4.5)
Race				<.001
White	10,256 (59.2)	8992 (60.6)	1264 (50.8)
Black	3001 (17.3)	2402 (16.2)	599 (24.1)
Hispanic	2590 (14.9)	2132 (14.4)	458 (18.4)
Asian/PI	630 (3.6)	554 (3.7)	76 (3.1)
Other	854 (4.9)	763 (5.1)	91 (3.7)
BMI, kg/m2				<.001
≤24.9	2558 (14.8)	2339 (15.8)	219 (8.8)
25-29.9	6787 (39.2)	5996 (40.4)	791 (31.8)
≥30	7529 (43.4)	6125 (41.3)	1404 (56.4)
Missing	457 (2.6)	383 (2.6)	74 (2.97)
Gestational age, wk				<.001
37.0-38.6	5605 (32.3)	4704 (31.7)	901 (36.2)
39.0-40.6	10,201 (58.9)	8,891 (59.9)	1310 (56.3)
41.0-41.6	1525 (8.8)	1248 (8.4)	277 (11.1)
Any hypertension				<.001
No	14,945 (86.2)	13,094 (88.2)	1851 (74.4)
Yes	2386 (13.8)	1749 (11.8)	637 (25.6)
Any diabetes mellitus				<.001
No	16,588 (95.7)	14,266 (96.1)	2322 (93.3)
Yes	743 (4.3)	577 (3.9)	166 (6.7)
Primary cesarean				<.001
No	14,133 (81.6)	12,475 (84.1)	1658 (66.6)
Yes	3198 (18.5)	2368 (15.9)	830 (33.4)
Hospital type				<.001
University	7538 (43.5)	6060 (40.8)	1478 (59.4)
Community, teaching	6715 (38.8)	5931 (39.9)	784 (31.5)
Community, nonteaching	3078 (17.8)	2852 (19.2)	226 (9.1)

Abbreviations: BMI, body mass index; PI, Pacific Islander.

Of the women in the sample, 49.4% had labor induced. Whereas 21.9% of women whose labors were induced received greater than 11,400 mU of synthetic oxytocin, only 7% of women whose labor was not induced received more than 11,400 mU of synthetic oxytocin during the course of their labor. A total of 3198 women (18.5%) experienced a primary cesarean birth, and the rate was higher in women whose labor was induced when compared with women whose labor was not induced (22.8% vs 14.2%, respectively).

Supporting Information: Table S1 contains further information about demographic data of women excluded because of incomplete oxytocin information. Compared with women with complete information, women who were excluded from the study because of missing oxytocin data were more likely to be younger than 20 years of age (29.7% vs 18.9%, P <.001) and Black (31.2% vs 17.3, P <.001), and all gave birth at university teaching hospitals. Although 18.5% of women with complete oxytocin information had a primary cesarean, that percentage was 27.1% among women with incomplete oxytocin information (P < .001).

The median total dose of oxytocin in this sample was 3854 mU, whereas the mean total dose was 5882.1 mU (SD, 6420.6 mU). In this sample, 86% of women were exposed to 2 to 11,400 mU of oxytocin and the remaining 14% were exposed to greater than 11,400 mU. The 11,400 mU threshold fell between the 75th and 90th percentiles for oxytocin exposure, so at least 75% of women in the sample received less than 11,400 mU of oxytocin.

The adjusted regression controlled for oxytocin total dose, age, race, BMI, gestational age, any hypertension, any type of diabetes, and hospital type. In all regressions, women were clustered by the attending physician identifier to control for variation in provider practice. In the adjusted regression model, women who received more than 11,400 mU oxytocin in labor had a 1.6 times greater odds of cesarean birth when compared with women who received less than or equal to 11,400 mU (aOR, 1.6; 95% CI, 1.01-2.6) (Table 2). Compared with women with a BMI in the normal weight category, women who were overweight had an increased odds of primary cesarean birth when other variables within the model were controlled for (aOR, 1.4; 95% CI, 1.2-1.7). These odds increased to 2.7 (95% CI, 2.3-3.2) for women with a BMI in the obese category. Women who gave birth at more than 41 weeks’ gestation had a greater odds of primary cesarean birth than did women at full term (aOR, 1.6; 95% CI, 1.2-2.1). Women older than 35 years had a higher odds of a primary cesarean birth (aOR, 2.0; 95% CI, 1.7-2.4), and women younger than 20 years were less likely to have primary cesarean birth (aOR, 0.6, 95% CI, 0.5-0.7), compared with women 20 to 34 years old.

Table 2.

Effects of Oxytocin Total Dose (mU) on Primary Cesarean Birth^a

Variables	Unadjusted OR (95% CI)	Adjusteda OR (95% CI)
Oxytocin total dose, mU
≤11,400	Ref	Ref
>11,400	2.6 (2.3-3.0)	1.6 (1.01-2.6)
Age, y
≤19	0.8 (0.7-0.98)	0.6 (0.5-0.7)
20-34	Ref	Ref
≥35	2.1 (1.8-2.5)	2.03 (1.7-2.4)
Race
White	Ref	Ref
Black	2.2 (1.9-2.5)	1.4 (1.1-1.8)
Hispanic	1.5 (1.3-1.7)	1.2 (1.0-1.5)
Asian/PI	2.0 (1.6-2.5)	1.8 (1.4-2.3)
Other	0.8 (0.6-0.95)	0.9 (0.7-1.1)
BMI, kg/m2
≤24.9	Ref	Ref
25-29.9	1.6 (1.3-1.8)	1.4 (1.2-1.7)
≥30	3.3 (2.9-3.9)	2.7 (2.3-3.2)
Missing	2.3 (1.7-3.1)	1.7 (1.2-2.3)
Gestational age, wk
37.0-38.6	0.96	0.8 (0.7-0.9)
39.0-40.6	Ref	Ref
41.0-41.6	1.6 (1.2-2.04)	1.6 (1.2-2.1)
Any hypertension
No	Ref	Ref
Yes	1.7 (1.4-1.9)	1.3 (1.1-1.5)
Any diabetes mellitus
No	Ref	Ref
Yes	2.2 (1.9-2.6)	1.7 (1.4-2.1)
Hospital Type
University, teaching	Ref	Ref
Community, teachingb	0.4 (0.3-0.5)	0.4 (0.3-0.6)
Community, nonteachingb	0.5 (0.4-0.7)	0.6 (0.4-0.9)

Abbreviation: BMI, body mass index; mU, milliunits; OR, odds ratio; P/I, Pacific Islander; Prob, probability.

^aThe adjusted regression controlled for oxytocin total dose, age, race, BMI, GA, any HTN, any DM, and hospital type. In all regressions, women were clustered by the attending physician identifier in order to control for variation in provider practice.

^bIn this sample, all community hospitals belonged to the same Consortium for Safe Labor site (ie, same health system).

Black, Hispanic, and Asian or Pacific Islander women had greater odds of a primary cesarean birth than white women (Table 2). In addition, any form of hypertensive or diabetic disorder increased the odds of having a primary cesarean (aOR, 1.3; 95% CI, 1.1-1.5; and aOR, 1.7; 95% CI, 1.4-2.1, respectively). Finally, the odds of having a primary cesarean birth were lower at the community hospitals in the study, which were all a part of the same hospital system, when compared with the university teaching hospitals.

Among women who did not have their labor induced, those who were exposed to more than 11,400 mU of synthetic oxytocin in labor were 2.7 (95% CI 1.3-5.6) times more likely to have a cesarean birth, compared with those who were exposed to less than 11,400 mU of synthetic oxytocin. In women whose labor was induced, those exposed to greater than 11,400 mU of synthetic oxytocin in labor were not more likely to have a cesarean birth, compared with those exposed to less than 11,400 mU (aOR 1.2; 95% CI 0.7-2.1).

DISCUSSION

This study found that total dose of oxytocin higher than 11,400 mU was associated with an increased odds of primary cesarean birth in women with an NTSV pregnancy. Although there was no a priori hypothesis as to what the threshold might be, the dose was surprisingly high. Indeed, in this data set, women who were exposed to 8096 mU and 13,628 mU were in the 75th and 90th percentile for exposure, respectively. There is a known association (although not a causal link) between oxytocin exposure in labor and an increased risk of cesarean birth. This study adds to what is known by examining the total dose of oxytocin, as opposed to a binary variable for whether or not a woman was exposed to the medication during labor.

Exposure to greater than 11,400 mU of oxytocin did not increase the odds of cesarean birth among women whose labor was induced. For women whose labor was not induced, receipt of more than 11,400 mU of oxytocin was associated with a significantly increased odds of cesarean birth. For women whose labors were not induced, exposure to oxytocin during labor was likely for augmentation. Oxytocin is used to augment labor when labor progress is slower than expected. Thus, the large exposure to oxytocin in this subgroup likely suggests these women experienced labor complications such as dystocia, malposition, or cephalopelvic disproportion that are known to be associated with an increased incidence of cesarean birth.

Although previous studies have found an association between oxytocin and cesarean birth, no study, to the best of our knowledge, has reported an oxytocin threshold at which the risk for primary cesarean birth increases. Labor induction and augmentation with oxytocin have both been identified as risk factors for cesarean birth among women with an NTSV pregnancy in retrospective or prospective cohort studies.^30-32 No single study is sufficient to prove causality. Both longitudinal cohort studies and randomized controlled trials may be used to build a case for causality. The randomization, which is a hallmark of the randomized controlled trial, is a robust way to control for bias but does not completely eliminate bias from a study. The recent “Randomized Trial of Induction versus Expectant Management” (ARRIVE) randomized low-risk nulliparous women to labor induction between 39 0/7 and 39 4/7 weeks’ gestation or expectant management.³³ Low-risk in the ARRIVE trial was defined as not having any maternal or fetal condition that was an indication for birth before 40 5/7 weeks’ gestation.³³ The primary outcome of interest in the study was a composite of perinatal death or severe neonatal complications and the secondary outcome was cesarean birth. The authors reported that women whose labor was induced had an 18.6% cesarean rate compared with a 22.2% cesarean rate among women who were managed expectantly (also reported as relative risk, 0.84; 95% CI, 0.76-0.93).³³ No guideline for labor induction was prescribed by the ARRIVE trial researchers for either arm (induction or expectant management) of the study, so conclusions about the actual effect of induction and specific induction agents on cesarean rates from this study are not easy to determine.³³ The heterogeneity of studies examining the association between induction or augmentation and perinatal outcomes, and the wide array of procedures or medications used for induction and augmentation, can make distilling conclusions to guide clinical practice difficult.

This study’s results are consistent with the literature that primary cesarean birth in women with an NTSV pregnancy is a complex phenomenon and that oxytocin exposure is associated with an increased odds of cesarean birth. One pathway by which oxytocin exposure might be associated with cesarean birth is via oxytocin’s effect on uterine contractions. For example, oxytocin can result in contractions that are too frequent or long, interfering with uteroplacental perfusion and leading to Category II/III fetal heart rate patterns, which in turn become an indication for cesarean. Conversely, a high total oxytocin dose may also be a flag for an underlying condition that is not identified clinically, such as a true cephalopelvic disproportion.

The association between increased BMI and both increased oxytocin exposure and increased odds of cesarean birth found in this study has been noted elsewhere.^5,22,34,35 BMI in the obese category is one of the top predictors of the likelihood of cesarean birth.^5,35 Additionally, women with a BMI in the obese category are exposed to greater rates of oxytocin in the first stage of labor, compared with women with a BMI in the normal category, and are more likely to receive a greater cumulative dose of oxytocin.^22,34 In the current study, women with a BMI in the obese category were more likely to be exposed to greater than 11,400 mU of oxytocin, compared with normal weight women, and also had a greater odds of cesarean birth. When the interaction between BMI and oxytocin was examined, however, exposure to greater than 11,400 mU of oxytocin (compared with <11,400 mU) within each BMI category did not have a statistically significant effect on the odds of a cesarean.

Black women with an NTSV pregnancy in this sample had an increased odds of having a cesarean compared with white women, even when adjusting for the other variables in the model. This increased odds of cesarean birth among Blackwomen has been reported in many other studies that have controlled for maternal, perinatal, and system risk factors.^6,36-38 Black women are also more likely to receive a cesarean for having Category II/III fetal heart rate patterns, whereas white women are more likely to have a cesarean for failure to progress.^6,37

One clinical implication of the finding that higher doses of oxytocin are associated with a higher odds of cesarean might be to find ways to decrease oxytocin total dose exposure by discontinuing oxytocin for women in active labor.^39,40 A Cochrane review of discontinuing oxytocin in the active phase of labor found that stopping oxytocin in active labor (defined as well-established contractions and >5 cm dilatation) was associated with a decreased cesarean birth rate (relative risk, 0.69; 95% CI, 0.56-0.86), although this was low-certainty evidence.³⁹ Another systematic review and meta-analysis on the same topic, using the same definition of active labor, also reported a reduction in cesarean birth rates when oxytocin was discontinued in active labor.⁴⁰ Another clinical implication is to develop or revise oxytocin administration guidelines. Studies that have assessed the impact of implementing oxytocin guidelines typically find decreased oxytocin use and improved patient outcomes but report mixed findings as to whether the intervention reduces cesarean birth rates.^41-44

Most hospitals have an oxytocin guideline to encourage safety and uniformity in practice. These guidelines typically include starting rates, rate and time increments for titrating the medication, and maximum rates. These guidelines do not necessarily include addressing the overall length of time or total dose of oxytocin a woman is exposed to. This study suggests that clinicians might consider attending to the cumulative dose of oxytocin a woman receives as an indicator of risk.

This study has limitations. Although the CSL hospital sample was designed to be representative of the United States, the sample in this secondary analysis excluded 4 of 9 geographic regions because of no oxytocin dose data. There are some system variables, such as hospital nursing resources, hospital size, and teaching status, that were not available and might be possible confounders. Other potential confounders or mediators of the association between oxytocin and cesarean, such as malposition, labor stage at oxytocin initiation, and duration of labor, were not controlled for. The positive association between increased oxytocin dose and increased odds of cesarean may represent an underlying etiology not measured in this study. Future studies should include these variables. Although the type of hospital where a woman gave birth was controlled for in the regression model, all of the community hospitals were from the same system. The results regarding primary cesarean birth in university versus community hospitals, therefore, should be interpreted with caution. The actual incidence of primary cesarean births was lower in the analytic sample than was expected. Our sample size, however, was almost 5 times what the a priori power analyses dictated. The BMI variable was based on admission, instead of prepregnancy, weight, so comparison with BMI in other studies should be cautious. Women who were excluded from the sample because of incomplete oxytocin dose information all gave birth at university hospitals and were more likely to be younger than 20 years old, Black, receive less oxytocin, and have cesareans. As a result, our sample may not fully represent all women who received oxytocin.

CONCLUSION

Oxytocin exposure exceeding 11,400 mU is associated with an increased odds of primary cesarean in nulliparous women at term who have a singleton pregnancy and vertex presentation. Although the oxytocin threshold identified in this study is quite high, attention to the total dose of oxytocin a woman is exposed to during labor may assist clinicians in identifying those who are at risk for cesarean birth.

These findings also suggest there is a gap in knowledge regarding best practice for managing labor for women with obesity. Current practice frequently employs oxytocin to stimulate contractions. There is a need for research to find alternative ways of supporting women who have a BMI in the obese categories who may respond differently to oxytocin exposure. These results point to a need for precision medicine and an ability to tailor labor care for an individual woman.

Supplementary Material

Supplementary Table

Table S1. Comparison of Women with Complete Synthetic Oxytocin Dose Information (N = 17,333) to Women with Incomplete Dose Information (n = 2498)

Quick points.

• In women who are nulliparous with a term, singleton fetus in vertex presentation (NTSV), the odds of cesarean birth increase 60% with exposure to greater than 11,400 mU of oxytocin during labor.

• Among women who were NTSV, Black women are 1.4 times more likely to have a cesarean birth than white women, even when controlling for patient risk factors, health care provider, and hospital, indicating a significant racial disparity requiring attention.

• A woman’s risk status changes over the course of labor, and conversations about the nature of that changing risk, what it means, and options should be encouraged.

• There is a gap in knowledge regarding best practice for managing laboring women who are obese, as current practice relies heavily on oxytocin and continuous monitoring, despite the understanding that these women may have a different response to oxytocin and are more difficult to monitor than are women with a normal body mass index.