Are there ethnic differences in the length of labor?
Mara B. Greenberg, MD, Yvonne W. Cheng, MD, MPH, Linda M. Hopkins, MD,
Naomi E. Stotland, MD, Allison S. Bryant, MD, MPH,
Aaron B. Caughey, MD, MPP, MPH
Department of Obstetrics, Gynecology, and Reproductive Sciences, University of California, San Francisco, CA
Received for publication March 10, 2006; revised May 26, 2006; accepted June 2, 2006
KEY WORDS
Length of labor
Ethnicity
Race
Objective: The purpose of this study was to examine the lengths of the first and second stages
of labor among different racial/ethnic groups to determine whether different norms should be
established.
Study design: This was a retrospective cohort study of all laboring, term, singleton, vertex deliv-
eries in a single academic institution. Median lengths of first and second stages of labor were com-
pared among 4 racial/ethnic groups: black, Asian, white, and Latina. Kruskal-Wallis, Wilcoxon
rank sum tests, and multivariate linear and logistic regression models were performed.
Results: In 27,521 births, the lengths of first stage of labor did not differ significantly among
groups in the multivariate analysis. In the second stage of labor, black women had shorter labors,
both overall and stratified by epidural use. In the multivariate analysis, when controlled for de-
mographics, parity, epidural, chorioamnionitis, birthweight, delivery year, and labor manage-
ment, black women had a shorter second stage than did white women (nulliparous women, 22
minutes; multiparous women, 7.5 minutes; P ! .001) and lower rates of prolonged second stage
(odds ratio, 0.6; P ! .001). Nulliparous Asian women had a significantly longer second stage and
higher rates of prolonged second stage, and nulliparous Latina women had a shorter second
stage, compared with nulliparous white women.
Conclusion: When data are controlled for confounding factors, black women had a shorter length
of second stage of labor than did women in other ethnic groups. These differences appear to be
clinically significant. This contributes to the support of a multifactorial redefinition of labor
curves, which are used widely in the management of labor.
Ó 2006 Mosby, Inc. All rights reserved.
The definition and description of labor as a clinically
important area of investigation is as compelling today as
it was 50 years ago when Emmanuel Friedman
first
published analyses of labor curves that were generated
from his study of a group of 100 white women. Of
particular interest is that the cesarean delivery rate in-
creased to 29.1% in 2004, the highest ever in the United
States,
with the indication of failed progress of labor re-
sponsible for a large proportion of primary cesarean de-
liveries.
Thus, our ability to understand the factors that
Supported in part by the National Institute of Child Health and Human Development, Grant # HD01262 as a Women’s Reproductive Health
Research Scholar (A.B.C.; N.E.S.).
Presented at the 26th Annual Meeting of the Society for Maternal Fetal Medicine, January 30-February 4, 2006, Miami, FL.
Reprints not available from the authors.
0002-9378/$ - see front matter
Ó 2006 Mosby, Inc. All rights reserved.
doi:10.1016/j.ajog.2006.06.016
American Journal of Obstetrics and Gynecology (2006) 195, 743–8
contribute to progress in labor has wide-ranging impli-
cations for not only the standards of diagnosis and man-
agement of labor, but also for subsequent pregnancies
that may be at increased risk because of a previous ce-
sarean delivery.
Despite the growing body of evidence that labor
norms (as previously described by Friedman
and
others) may not fit many current obstetric populations,
the standard curve still is used widely to guide labor
interventions. For example, Albers
found (in a 1999
descriptive study of spontaneous, term, vertex labor in
women without epidural analgesia) that both multipa-
rous and nulliparous labors were considerably longer
than those described in Friedman’s populations. Nota-
bly in the second stage, the women studied by Albers
had lengths twice as long as those described by Fried-
man for both nulliparous women (54 vs 33 minutes)
and multiparous women (18 vs 8.8 minutes). Similar
studies performed by others, including Kilpatrick
and
Zhang et al,
again found lengths of spontaneous labor
to be longer than those described in Friedman’s cohort,
potentially because of differences in more contemporary
labor management and demographic characteristics.
Although these studies describe normal labor curves
in both nulliparous and multiparous women, few stud-
ies have attempted to analyze the data with respect to
ethnicity.
One of these previous studies examined
length of labor by ethnicity and found that, between His-
panic, non-Hispanic white and Native American women,
differences were limited to the second stage and included
shorter times for Native American nulliparous women.
Of note, studies that have examined the association be-
tween ethnicity and perinatal outcomes other than length
of labor have found significant differences that include
length of gestation and rates of prematurity, perineal lac-
erations, gestational diabetes mellitus, and preeclampsia
among other clinically important entities.
Given this background and the dearth of studies that
have examined length of labor by race/ethnicity, our
objective was to examine length of labor among differ-
ent racial/ethnic groups. Although this type of descrip-
tive study does not speak to causality, it is aimed at
contributing to the developing description of rate of
progress in labor and the associations on which practi-
tioners can draw when considering labor management.
Methods
We conducted a retrospective cohort study of all labor-
ing, term, singleton vertex deliveries between 1976 and
2001 at the University of California, San Francisco.
Race/ethnicity was self-reported by the patients at their
first prenatal visit. We excluded all women without a
reported ethnicity or ethnicity that did not fall into the
4 major racial/ethnic groups that we examined: black,
Asian, white, and Latina women. Median lengths of the
first and second stages of labor were compared between
these 4 major racial/ethnic groups. In addition, compar-
isons were made of the proportions of women who had a
first stage of labor of O18 hours, a second stage of labor
of O3 hours, and a prolonged second stage of labor. The
latter was defined as O3 hours for nulliparous women
with epidural analgesia, O2 hours for multiparous
women with epidural analgesia, O2 hours for nullipa-
rous women without epidural analgesia, and O1 hour
for multiparous women without epidural analgesia, as
described in the American College of Obstetricians and
Gynecologists Practice Bulletin No 49.
The onset of
the first stage of labor was defined as the time when reg-
ular, painful contractions that led to cervical change be-
gan occurring every 3 to 5 minutes, per patient history.
The second stage of labor was defined as the time be-
tween complete cervical dilation and the delivery of the
infant. Women for whom this information was not avail-
able were excluded from the study.
Statistical comparisons were made with the use of the
Kruskal-Wallis and Wilcoxon rank sum tests. Further,
the lengths of first and second stages of labor were
compared with the use of survival analysis. Multivariate
linear and logistic regression models were used to
control for potential confounders that included demo-
graphic characteristics, parity, use of regional anesthe-
sia, chorioamnionitis, length of gestation, birthweight,
year of delivery, body mass index, Medicaid insurance
status, fetal occiput posterior (OP) position, year of
delivery, intrapartum cesarean delivery, and labor man-
agement techniques that included the use of oxytocin,
artificial rupture of membranes, and prostaglandins for
labor augmentation or induction. Body mass index was
calculated with the height and weight at the first prena-
tal visit. Fetal position was determined at the time of
delivery of the fetal vertex. Chorioamnionitis was de-
fined as maternal temperature if R38.0(C, plus R1 of
the following: maternal tachycardia, fetal tachycardia,
uterine tenderness, or purulent amniotic fluid. Addi-
tionally, subgroup analyses were conducted in women
with and without the induction of labor, epidural use,
and cesarean delivery. Because of concern about using
standard linear regression to analyze the length of labor
data, which does not have a normal distribution, the
models were run as log-transformed data also. Two-
tailed probability values of !.05 were considered sta-
tistically significant. Institutional review board approval
was obtained from the Committee on Human Research
at University of California, San Francisco.
Results
Overall, there were 28,793 women with a term, singleton
cephalic labor who self-identified as 1 of the 4 racial/
ethnic groups in the study. Of these, 1272 women (4.4%)
did not have information available on the length of
744
Greenberg et al
labor. Thus, there were 14,719 nulliparous and 12,802
multiparous women who met study inclusion and exclu-
sion criteria. Of these women, 13.5% were black, 30.5%
Asian, 45.0% white, and 11.0% Latina (
). For
both first and second stages of labor, there were
statistically significant differences between the median
lengths of labor of the different ethnic groups (
). For the first stage of labor, nulliparous black women
had a median length of 9.5 hours compared with 10.0 to
10.3 hours among women in the other 3 ethnic groups
(P = .01). Fewer black women had a first stage of la-
bor O18 hours (13.8% compared with 16.4% for nullip-
arous white women; P = .03). For first-stage data that
were analyzed by epidural status, nulliparous black
women with epidural analgesia were significantly less
likely than white women to labor for O18 hours (17.1%
vs 21.9%; P ! .001). However, the percentage of nullipa-
rous women without epidural analgesia who had first
stage of labor for O18 hours did not differ significantly
between ethnic groups. These first-stage parameters did
not differ significantly for multiparous women.
In the second stage of labor, black women had
shorter times for both nulliparous women and multip-
arous women than did women in the other ethnic
groups. Compared with white women, black women
had median second-stage lengths of 50 versus 92 minutes
(P ! .001) and 15 versus 20 minutes (P ! .001) for nul-
liparous women and multiparous women, respectively.
Nulliparous black women, multiparous black women,
and nulliparous Latina women had shorter second
stages than white women, regardless of epidural status
(
;
). Black women also had lower
rates of second stage of labor of O3 hours in nulliparous
women (8.5% vs 19.8%; P = .003) and multiparous
women (2.3% vs 3.7%; P ! .001) and had lower rates
of prolonged second stage of labor. These differences
in rates of second stage of O3 hours and prolonged sec-
ond stage persisted across subgroup analysis by epidural
status for both nulliparous women and multiparous
women.
In the multivariate analyses that controlled for po-
tential confounders, there were no differences between
the racial/ethnic groups in the length of first stage of
labor. However, many second-stage differences persisted
after being controlled for demographics, parity, epidural
use, chorioamnionitis, birthweight, body mass index, OP
position, year of delivery, intrapartum cesarean delivery,
and labor management (
). Black women had
second stages 9.4 minutes shorter for nulliparous women
without epidural analgesia (P = .002) and 30 minutes
shorter for nulliparous women with epidural analgesia
(P ! .001), as compared with white women, with
smaller differences noted among multiparous women.
Asian women had longer second stages than white
women by 5.2 and 5.9 minutes for nulliparous women
with and without epidural analgesia, respectively, and
8.2 minutes for multiparous women with epidural anal-
gesia. When induced/augmented labors were separated
from spontaneous labors, nulliparous black women in
both subgroups had persistently shorter second stages,
whereas only spontaneously laboring black multiparous
Table I
Demographics and intrapartum characteristics
Variable
Black women
Asian women
White women
Latina women
P value
Nulliparous (n)
1793
4383
7137
1406
Multiparous (n)
1961
4038
5238
1565
Gestational age R41 wk (%)
25.0
22.6
32.3
20.5
!
.001
Birthweight R4 kg (%)
7.6
8.1
16.1
12.7
!
.001
Maternal age R35 y (%)
6.7
14.5
18.8
10.5
!
.001
Induction of labor (%)
15.0
10.5
14.4
15.4
!
.001
Epidural (%)
52.5
46.3
51.6
49.3
!
.001
Medicaid insurance (%)
44.2
38.8
39.9
38.8
!
.001
OP position (%)
5.7
3.7
4.5
3.7
!
.001
Body mass index O29 kg/m
2
(%)
21.2
3.5
7.0
12.1
!
.001
Chorioamnionitis (%)
7.5
7.8
5.3
7.7
!
.001
Intrapartum cesarean delivery (%)
11.8
11.5
11.7
12.4
.580
Table II
Univariate analyses and median lengths of first and
second stages of labor
Variable
Black
women
Asian
women
Latina
women
White
women
Stage 1 (hr)
9.5
10.0
10.3
10.0
Multiparas
y
6.0
5.7
6.3
5.9
Stage 2 (min)
Nulliparas: Total
y
50
93
77
92
With epidural
142
137
Without epidural
53
45
54
Stage 2 (min)
Multiparas: Total
y
15
19
20
20
With epidural
54
49
45
Without epidural
15
All comparisons are to white women.
* P % .01.
y
P = .001.
Greenberg et al
745
women had significantly shorter second stages than
white women (
). When the regression models
were conducted in women with and without cesarean
deliveries, the findings among those women without a
cesarean delivery were similar to the overall group. In
the small subgroup of women who were delivered by ce-
sarean, significant differences persisted in the second
stage only for nulliparous black women, as compared
with white women (
). Those differences re-
ported earlier were of similar direction and statistical
significance when analyses were conducted with the
log-transformed lengths of labor (data not shown).
When the second stage of labor of O3 hours and
prolonged second stage were examined, both continued
to show significant differences between the racial/ethnic
groups (
). When the rates of second stage of
O
3 hours were analyzed by epidural subgroups in the
multivariate analysis, black women with epidural anal-
gesia had lower rates than white women, and Asian
women with epidural analgesia had higher rates than
white women, regardless of parity, although no differ-
ences were demonstrated among women without epidu-
ral analgesia. Of note, to address possible trends in labor
management over time, a subgroup analysis limited to
deliveries within the last 10 years was also performed;
similar outcomes were found (data not shown).
Comment
We have demonstrated a statistically significant differ-
ence in the length of labor in a large multiethnic cohort
of women. When we controlled for confounding factors,
black women had shorter lengths of second stage of
labor, and Asian women had longer lengths, as com-
pared with white women. These differences were also
demonstrable when we considered the rates of a second
stage of O3 hours and prolonged second stage. The
most clinically significant differences in the second stage
appear to be for nulliparous women, with notable vari-
ance by epidural status.
In consideration of possible etiologic factors for dif-
ferences in the length of labor between women of different
racial/ethnic groups, several categories present them-
selves as possible candidates: pelvic anatomy and poten-
tially subsequent fetal position, fetal and maternal size,
other factors related to maternal habitus, factors indica-
tive of general maternal health, or iatrogenic factors such
as labor management or the differential effect of epidural
analgesia on the progress in labor. It is well documented
that birthweight differs markedly by ethnicity within the
United States, even among ethnic groups by country of
origin, and that after controlling for the length of gesta-
tion black women have smaller babies per gestational age
at term than do white women.
Studies of pelvimetry
among women of different racial/ethnic backgrounds
have demonstrated variance in pelvic shape and even pel-
vic floor area, and as a corollary to this, differences in rates
of OP position between groups.
In the current study, we
found that black women were less likely than women in
other ethnic groups to have infants with birthweight
O
4000 g and were more likely to have fetal OP position.
However, we did control for both of these factors that the-
oretically could have opposing effects on the time-course
of labor, especially in the second stage. Another possible
cause that is not biologically based is that women of differ-
ent races/ethnicities are treated differently by clinicians.
For example, perhaps women in the second stage differen-
tially receive interventions intended to effect delivery
sooner, among women of different races/ethnicities. Al-
though this may be true, even when we excluded women
who underwent cesarean delivery, the differences between
the lengths of second stage persisted.
Our findings have potential impact in contributing to
the understanding of factors that affect length of labor
Figure 1
Kaplan-Meier survival curve: Nulliparous women
with epidural analgesia in the second stage of labor; percentage
of women who were delivered over time.
Figure 2
Kaplan-Meier survival curve: Nulliparous women
without epidural analgesia in the second stage of labor; per-
centage of women who were delivered over time.
746
Greenberg et al
and subsequently labor assessment and management
parameters. These data will likely prove most useful in
the context of the study of other maternal, fetal, and
gestational characteristics that may affect labor. Such an
endeavor can contribute to the creation of a composite-
expected profile of an individual’s labor. Thus, rather
than the current 1-size-fits-all approach to labor norms,
such norms might be based on a woman’s race/ethnicity,
age, height, weight, gestational age, epidural use, and
parity. That we should consider modifying and possibly
extending our limits of the definition of prolonged labor
has merit, and at least 1 group of investigators has demon-
strated that more vaginal deliveries will be achieved
without demonstrable neonatal injury by such an exten-
sion.
Furthermore, it has also been demonstrated that
the extension of the limits of the second stage of
labor to O3 hours will lead to further spontaneous
vaginal deliveries with no difference in neonatal out-
comes, but some increase in maternal outcomes such as
perineal trauma.
On the basis of these types of results
and on our findings, we believe that similar future stud-
ies should examine labor norms with relation to these out-
comes by ethnicity to determine whether varying cutoffs
should be used.
Our study is not without limitations. First, our
examination of race/ethnicity as a predictor of length
of labor may be susceptible to confounding bias. We
attempted to control for many identifiable confounders
using multivariate regression models. However, there
may be other confounders of which we did not conceive
or include in our model. Missing information bias is
another possible limitation of the study. Although only
Table IV
Second stage of labor, multivariate logistic regression analyses
Variable
Black women
Asian women
Latina women
Stage 2 O 3h
Nulliparas
0.50 (0.41-0.62)
1.16 (1.04-1.30)
0.94 (0.79-1.12)
Multiparas
0.55 (0.40-0.79)
1.09 (0.87-1.38)
0.92 (0.70-1.26)
Prolonged Second Stage
Nulliparas
0.54 (0.45-0.68)
1.19 (1.08-1.32)
0.94 (0.80-1.10)
Multiparas
0.61 (0.49-0.05)
1.14 (0.99-1.31)
1.00 (0.82-1.21)
* Data are presented as odds ratio (95% CI); controlled for induction of labor, maternal age, gestational age, Medicaid status, birthweight, OP
position, body mass index, intrapartum cesarean delivery, year of delivery, and chorioamnionitis; comparison group is white.
Table III
Second stage of labor: Multivariate analyses
Variable
Black women
Asian women
Latina women
Nulliparas
22.0 (26.9, 17.1)
C
5.0 (C1.6, C8.3)
6.8 (11.9, 1.7)
Stratified
With epidural
y
30 (37.3, 22.8)
C
5.2 (C0.1, C10.2)
9.7 (17.2, 2.1)
Without epidural
y
9.4 (15.3, 3.4)
C
5.9 (C2.0,C9.9)
1.8 (7.9, 4.3)
Induction of labor
z
22.3 (34.2, 10.4)
C
7.4 (
1.9, C16.7)
4.7 (17.2, C7.7)
Spontaneous labor
z
21.4 (26.8, 16.1)
C
4.4 (C0.8, C8.0)
7.1 (12.7, 1.6)
With cesarean
x
36.6 (67.5, 5.6)
5.9 (23.8, C12.0)
19.2 (44.9, C6.4)
Without cesarean
x
20.8 (25.6, 16.0)
C
5.9 (C2.5, C9.2)
5.8 (10.9, 0.7)
Multiparas
7.5 (11.4, 3.7)
C
1.8 (
1.1, C4.8)
C
0.8 (
3.3, C4.8)
Stratified
With epidural
y
5.9 (13.9, C2.0)
C
8.2 (C1.6, C14.8)
C
5.4 (
3.1, C13.9)
Without epidural
y
7.6 (11.6, 3.6)
1.2 (4.1, C1.8)
1.1 (5.2, C3.0)
Induction of labor
z
C
0.8 (
9.5, C11.1)
C
8.6 (
0.3, C17.4)
C
6.8 (
3.7, C17.2)
Spontaneous labor
z
8.7 (12.8, 4.5)
C
1.1 (
2.0, C4.2)
0.1 (4.5, C4.3)
With cesarean delivery
x
C
14.2 (
43.2, C71.6)
C
12.8 (
25.5, C51.1)
C
1.6 (
46.7, C50.0)
Without cesarean delivery
x
7.9 (11.7, 4.1)
C
1.7 (
1.1, C4.6)
C
1.2 (
2.7, C5.2)
Data are presented as difference in minutes, compared with the white group (95% CI).
* Controlled for epidural, induction of labor, maternal age, gestational age, Medicaid insurance status, birthweight, OP position, body mass index,
intrapartum cesarean delivery, year of delivery, and chorioamnionitis.
y
Controlling for all of the aforementioned confounders, except epidural status.
z
Controlling for all of the aforementioned confounders, except the use of augmentation or induction of labor that included pitocin, prostaglandin
analogues, and artificial rupture of membranes.
x
Controlling for all of the aforementioned confounders, except cesarean delivery status.
Greenberg et al
747
4.4% of women did not report the onset of labor, this
does limit the study findings and implications solely to
those women who can report such information. Another
possible issue is that of generalizability, given that our
population was drawn from a single institution. It may
be that there are differences that were seen in our local
population that may not exist in other populations.
Further, labor management was performed by a small
group of providers at a single institution. Although we
controlled for augmentation of labor, we had no way to
control for how aggressively such augmentation was
used. Such management style should not differ by race/
ethnicity, and it likely almost certainly does in many
instances, thereby leading to differences in length of
labor. We attempted to control for such differences in
our second-stage models by creating a subgroup analysis
of induction versus spontaneous labor. Notably the
differences among black women persisted for both
subgroups. This suggests that, if there is such a bias,
the effect (at least in this cohort of women) was minimal.
Despite these limitations, the present analysis con-
tributes to the growing body of evidence that supports
multifactorial redefinition of labor curves. Direction for
future characterization of ethnic differences in the length
of labor includes prospective data collection in a similar,
population-representative cohort and includes the rate
of cervical change, examination of latent and active
phase of the first stage of labor, and examination of
perinatal outcomes by varying length of labor thresh-
olds among different racial/ethnic groups.
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