Incidence of Incisional Hernia after Cesarean Delivery: A
Register-Based Cohort Study
Anna J. M. Aabakke
*, Lone Krebs
, Steen Ladelund
, Niels J. Secher
1Department of Obstetrics and Gynecology, University of Copenhagen, Holbæk Hospital, Holbæk, Denmark, 2Clinical Research Center, University of Copenhagen,
Hvidovre Hospital, Hvidovre, Denmark, 3The Research Unit Women’s and Children’s Health, The Juliane Marie Center, Copenhagen University Hospital, Copenhagen,
Denmark, 4Department of Obstetrics and Gynecology, Aarhus University Hospital, Aarhus, Denmark
To estimate the incidence of incisional hernias requiring surgical repair after cesarean delivery over a 10-year
This population- and register-based cohort study identified all women in Denmark with no history of previous
abdominal surgery who had a cesarean delivery between 1991 and 2000. The cohort was followed from their first until 10
years after their last cesarean delivery within the inclusion period or until the first of the following events: hernia repair,
death, emigration, abdominal surgery, or cesarean delivery after the inclusion period. For women who had a hernia repair,
hospital records regarding the surgery and previous cesarean deliveries were tracked and manually analyzed to validate the
relationship between hernia repair and cesarean delivery. Data were analyzed with a competing risk analysis that included
each cesarean delivery.
We identified 57,564 women who had had 68,271 cesarean deliveries during the inclusion period. During follow-up,
134 of these women had a hernia requiring repair. Of these 68 (51% [95% CI 42–60%]) were in a midline incision although
the transverse incision was the primary approach at cesarean delivery during the inclusion period. The cumulated incidence
of a hernia repair within 10 years after a cesarean delivery was 0.197% (95% CI 0.164–0.234%). The risk of a hernia repair was
higher during the first 3 years after a cesarean delivery, with an incidence after 3 years of 0.157% (95% CI 0.127–0.187%).
The overall risk of an incisional hernia requiring surgical repair within 10 years after a cesarean delivery was 2
per 1000 deliveries in a population in which the transverse incision was the primary approach at cesarean delivery.
Citation: Aabakke AJM, Krebs L, Ladelund S, Secher NJ (2014) Incidence of Incisional Hernia after Cesarean Delivery: A Register-Based Cohort Study. PLoS
ONE 9(9): e108829. doi:10.1371/journal.pone.0108829
Editor: Katariina Laine, Oslo University Hospital, Ulleva
Received April 28, 2014; Accepted June 13, 2014; Published September 30, 2014
Copyright: ß2014 Aabakke et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by grants from the Faculty of Health, University of Copenhagen (www.sund.ku.dk), The Region Zealand Health Sciences
Research Foundation (http://www.regionsjaelland.dk/sundhed/forskning/praktisk%20om%20forskning/sider/den-regionale-forskningsfond.aspx), and Depart-
ment of Obstetrics and Gynecology, University of Copenhagen, Holbæk Hospital. The funders had no role in study design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* Email: email@example.com
Cesarean delivery is the most common surgical procedure in the
United States, and the rates are increasing worldwide [1,2]. In the
United States, Latin America, Australia, and several European
countries cesarean delivery rates are presently above 30% [3–6].
Several studies have found both short- and long-term complica-
tions related to cesarean delivery [7–11]. Incisional hernia is a
well-known long-term complication of abdominal surgery, with a
reported incidence of 3.0–20.6% in association with midline
incisions and 0–2.1% with lower transverse incisions [12–19]. Two
meta-analyses have confirmed that the risk of developing an
incisional hernia after abdominal surgery is higher after a midline
than a transverse incision [20,21]. However, only one study, with 6
to 12 months follow-up, has focused on incisional hernia after
cesarean delivery . The authors found no incisional hernias
after 280 cesarean deliveries performed through a transverse
incision. Studies have found that more than 50% of incisional
hernias occur more than 1 year after surgery, indicating that the
incidence of incisional hernia after cesarean delivery might be
The objective of this study was to estimate the incidence of
incisional hernias requiring surgical repair after cesarean delivery
in a large cohort during a 10-year period.
Materials and Methods
This was a population-based cohort study with data obtained
from the Danish National Patient Register and the Danish
Medical Birth Register.
In Denmark all in- and out-patient hospital contacts are
registered in the Danish National Patient Register established in
1977 and all births in the Danish Medical Birth Register
established in 1968 [23,24]. Both registers were administered by
the Danish National Board of Health at the time of data
extraction. All Danish citizens have a unique personal identifica-
PLOS ONE | www.plosone.org 1 September 2014 | Volume 9 | Issue 9 | e108829
tion number, and information in the registers is linked to this
number. The registers contain information on age, gender,
surgical procedure, diagnosis, readmission, and reoperation, but
no surgical details (e.g. type of incision and suture material).
Surgical procedures have since 1996 been recorded based on
codes according to the NOMESCO classification of surgical
procedures (a common Nordic classification) . Before 1996,
surgical registrations were based on a national classification of
treatments and surgical procedures. The completeness of the
registration in the Danish National Patient Register is 98–100%,
and the validity of the system in terms of the type of surgical
procedure has been proven to be high [26–28].
This study was based on a search in the two registries. The main
inclusion criterion was a cesarean delivery between January 1,
1991 and December 31, 2000 in women with no history of
previous abdominal surgery. Women who had had a cesarean
delivery during the inclusion period were identified through the
codes for cesarean delivery (sectio_u and, sectio_f [1991–95], and
KMCA 10, 11, 12a, 12b [1996–2000]). The cohort was followed
from their first cesarean delivery until 10 years after their last
cesarean delivery in the inclusion period for an event of hernia
repair. Hernia repairs performed during 1991–95 were identified
by the codes herniotomia incisionalis (40680), herniotomia
ventralis (42400), herniotomia parietis abdominis (40600), hernio-
plastica cum implantation (40800), hernioplastica cum implanta-
tione laparascopica (40801), and herniotomia (40740, 42900, and
no previous diagnosis of inguinal hernia). Hernia repairs during
1996–2000 were identified by the code for surgeries of incisional
hernia (KJAD). We registered only the primary hernia repair, and
any recurrences were not included. We also registered events of
death, emigration, abdominal surgery, and cesarean deliveries
during the follow-up period. The surgical codes for hernia repair
do not contain information about the anatomical location of the
hernia. Therefore, for women with a hernia repair, hospital
records of the primary examination, description of surgery, and
discharge letter were retrieved as were the descriptions of the
previous cesarean deliveries. The records were manually analyzed
by two investigators (AA and LK) to validate the relationship
between hernia repair and cesarean delivery and to determine the
type of incision. When in doubt, consensus was obtained through
discussion. Before study initiation, the following exclusion criteria
for the hernia repairs were defined: Diastasis recti without hernia,
hernia not in the cesarean incision, and no hernia. Cases in which
no validation information was accessible were maintained in the
cohort as confirmed cases so that the incidence of hernia repair
was not falsely underestimated.
The results of the study were reported following the STROBE
recommendations for strengthening the reporting of observational
studies in epidemiology .
The study was approved by the Danish Data Protection Agency
(reg. no. 2013-41-2155). Ethical approval was not required.
Written informed consent was not required and therefore not
obtained for the clinical records to be used in the study. However,
patient records and the data-set were anonymized before analysis.
The cumulative risk of a hernia repair after cesarean delivery
was estimated by a competing risk analysis . Each cesarean
delivery during the inclusion period was analyzed individually.
The competing endpoints that mutually excluded each other were
hernia repair, death, abdominal surgery, and a consecutive
cesarean delivery. Emigration during the follow-up period caused
censoring. Right censoring also occurred after 10 years if the
woman did not meet any of the endpoints mentioned above.
Continuous data were analyzed by unpaired ttest when
normally distributed and by Mann Whitney test when not.
Nominal data were analyzed by chi
test or, when relevant,
Fischer’s exact test. Pvalues ,0.05 were regarded as statistically
The analyses were performed using R 3.0.2 (R Foundation for
Statistical Computing, Vienna, Austria) with the add-on library
cmprsk , and IBM SPSS Statistics 21 (SPSS Inc., Chicago, IL,
Between January 1991 and December 2000, 57,564 unique
women, with no history of previous abdominal surgery, had a total
of 68,271 cesarean deliveries. Of these, 158 were identified as
having a subsequent hernia repair performed within 10 years after
their cesarean delivery within the inclusion period. Analysis of the
hospital records excluded 24 cases. Of these, 4 women had a
history of previous abdominal surgery, 4 had abdominal surgery
after the cesarean delivery but before the hernia repair, one had
the hernia repair done coincidently with the including cesarean
delivery, 9 hernias were not in the cesarean incision, 5 had
diastasis recti without hernia, and 1 had no hernia. A total of 134
cases of hernia repair were included in the analysis. In 20 patients,
hospital records from the hernia repair surgery were not available,
primarily due to destruction of medical records after 10 years of
inactivity. In 68 cases (51% [95% CI 42–59%]) the hernias treated
were in a midline incision, in 50 cases (37% [95% CI 29–46%]) in
a transverse incision, and in 16 cases (12% [95% CI 7–19%]) in an
incision of unknown type due to missing cesarean delivery records.
Study design and flow including information on competing events
are illustrated in Figure 1. The basic characteristics of the hernia
repairs based on type of incision did not differ and are shown in
Table 1. There was no change over time in either the frequency of
cesarean deliveries resulting in hernia repairs or the distribution of
transverse and midline incisions among the hernias requiring
hernia repair (data not shown).
The cumulated incidence of a hernia repair within 10 years
after a cesarean delivery was 0.197% (95% CI 0.164–0.234%)
(Figure 2). The risk of a hernia repair was higher within the first 3
years after a cesarean delivery, with a cumulated incidence at 3
years of 0.157% (95% CI 0.127–0.187%) (Figure 2 and Table 2).
Among women who developed an incisional hernia requiring
surgical repair the median time from cesarean delivery to repair
was 1.36 years (IQR 0.79–2.27 years).
This prospective cohort study found a cumulated risk of 0.197%
of developing an incisional hernia requiring surgical repair within
10 years after a cesarean delivery. The risk of a hernia repair was
higher during the first 3 years after a cesarean delivery.
Less than half of the hernia repairs were performed within the
first year after the cesarean delivery (Table 2). Our study thereby
confirms previous studies showing that less than half of incisional
hernias develop within the first year after surgery . However, it
cannot be ruled out that women who developed an incisional
hernia within the first year after a cesarean delivery postponed
repair of various reasons.
Through validation, we found that the repairs identified in this
study were of hernias in both midline and transverse incisions and
that more than half of them were of hernias in midline incisions.
During the study period, the majority of cesarean deliveries in
Incisional Hernia after Cesarean Delivery
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Denmark were performed through a transverse incision, but,
unfortunately, the distribution of the two types of incisions during
the study period is unknown. We therefore can only try to make an
estimate of the risk of surgical repair of a hernia in transverse
incisions based on previously published data: In order not to
underestimate the risk, unknown incision types are assumed to be
transverse and the following assumptions regarding midline
incisions are made. The lowest found incidence of incisional
hernia in midline incisions is reported to be 3.0% in gynecological
surgeries . Additionally, one study found that 16% of
incisional hernias occurring within 10 years of laparotomy are
surgically repaired . Based on these findings, surgical hernia
repair was performed on 0.48% of all midline incisions in the study
population. Under these assumptions surgical repair of transverse
incision hernias was performed after 66 of 54.104 cesarean
deliveries during a 10 year period corresponding to an overall
crude incidence of 0.12%. The risk of a hernia in a transverse
incision requiring repair is therefore probably lower than the risk
found in this study. Previous studies have found that the risk of
developing a hernia in a midline compared with a transverse
incision is increased, with odds ratios of 1.68 to 3.33 [20,21]. This
difference most likely explains why more than half the hernia
repairs in our study were performed on hernias in midline
incisions, although the percentage of midline incisions in the
cesarean delivery cohort was low. We found no statistically
significant differences between the hernia repairs performed in a
midline compared with transverse incision regarding time from
cesarean delivery to repair, number of previous cesarean
deliveries, age at the cesarean delivery, or percentage performed
as an emergency procedure.
Factors other than the type of incision have been suggested to
influence the development of incisional hernias. The suture
material and suture technique used to close the fascia have been
shown to affect the risk of incisional hernia in midline incisions. A
Figure 1. Flow through the study. Number in brackets is number of excluded and censored cases found through validation. CD: cesarean
delivery. NA: Not available.
Table 1. Basic characteristics of surgical repairs performed on hernias in midline and transverse incisions.
Transverse incision Midline incision
(n = 50) (n = 68)
Age at last cesarean delivery before hernia repair (years) [Mean (SD)] 32 (5) 32 (5) 0.836
Number of cesarean deliveries before hernia repair [Median (IQR)] 1.5 (1.0–2.0) 2.0 (1.0–2.8) 0.139
Time from cesarean delivery to hernia repair (months) [Median (IQR)] 18.5 (9.6–26.0) 21.7 (11.4–60.8) 0.078
Acute hernia repair [n (%)] 2 (4.5%) 2 (3.1%) 1.000
Data analyzed with *unpaired t-test,
Mann Whitney test, and
Fischer’s exact test.
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suture technique with continuous sutures placed 1 cm apart and
1 cm from the incision using a suture 4 times the length of the
incision has been shown to prevent hernias . Additionally, a
meta-analysis found that slowly absorbable suture material caused
fewer hernias . Unfortunately, there are no studies on closure
techniques of the fascia in transverse incisions for cesarean
delivery. The risk of incisional hernia after cesarean delivery seems
to be so low that randomized trials with incisional hernia as the
primary outcome would need a very large number of participants
and at least 3 years of follow-up. And unfortunately, two current
multifactorial randomized trials with long-term follow-up after
cesarean delivery are not exploring methods to close the fascia
[33,34]. The development of incisional hernias may also be
influenced by factors such as BMI and post-cesarean complications
including infection. Unfortunately these variables were not
available in the Danish registers during the study period and
were not uniformly registered in the available medical records, and
could therefore not be included in this analysis. A case-control
study could theoretically evaluate differences in various risk factors
between cases with a hernia repair and a control group. However,
since data about BMI, post-operative infection, and surgical
technique for fascial closure were not consistently registered in the
records in the 1990s, the study would only be able to add
information about risk differences due to incision type and confirm
previously estimated odds ratios.
This is the largest study to date that gives an estimate of the risk
of developing an incisional hernia requiring repair after cesarean
delivery and the first study with a follow-up time of more than 1
year. However, the study has some limitations that need to be
considered. Register studies are dependent on the rate, complete-
ness, and accuracy of the registration. Both the registration rate
and the validity of type of surgical procedures in the Danish
National Patient Register have been shown to be high, and we
assume that our data are representative [26–28]. The surgical
codes for hernia repair do not contain information about
anatomical location of the hernia explaining the relatively high
rate of hernia repairs that were found to be unrelated to a previous
cesarean delivery through validation. We excluded women with
previous abdominal surgery from our cohort, and there may have
been some cases of hernia repairs after a cesarean delivery in this
group. However, it is unlikely that the incidence is different in the
excluded group compared with our cohort, and thus exclusion of
these women should not have affected our results.
Figure 2. Cumulative incidence of incisional hernias requiring repair after cesarean delivery. CD: cesarean delivery.
Table 2. Cumulative incidence of incisional hernias requiring surgical repair 1, 3, and 10 years after a cesarean delivery.
1 year 3 years 10 years
Hernia repair 0.072 (0.052–0.092) 0.157 (0.127–0.187) 0.197 (0.164–0.231)
Data are expressed as % (95% confidence interval).
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The Danish National Patient Register was established in 1977,
and patients with abdominal surgeries performed before 1977
could not be identified and excluded from our cohort. Addition-
ally, we excluded some cases of hernia repair in the process of
validation due to the identification of previous abdominal
surgeries. This may have caused us to underestimate the actual
incidence of hernia repair after cesarean delivery. But there are
also some factors that may have led to an overestimation of the
actual incidence of hernia repair after cesarean delivery. During
the process of validation, a number of hospital records were not
retrievable. We maintained these unconfirmed cases in the cohort
in order to avoid underestimation. Additionally, more than half
the repairs were of hernias in midline rather than transverse
incisions, the latter being the preferred incision for cesarean
delivery in the Western world . Unfortunately, the distribution
of the two types of incisions in Denmark in the 1990s is unknown,
although the transverse incision was the incision of choice. We
were therefore not able to separately estimate the risk of a hernia
repair in the two types of incisions.
The risk of developing an incisional hernia after a cesarean
delivery that does not require repair was not investigated in this
study. The validity of diagnostic codes in the Danish registries
have been shown to be poor [36,37] and we therefore selected the
surgical code of hernia repair. Medical treatment is free in
Denmark and we therefore assume that clinically important
incisional hernias were treated surgically due to the risk of serious
adverse events (e.g. incarceration). But small hernias may be left
un-treated, and thus the risk of an incisional hernia might be
higher than the risk of surgical hernia repair found in this study.
In conclusion, this study found that the overall risk of having an
incisional hernia requiring repair within 10 years of a cesarean
delivery was 2 per 1000 cesarean deliveries. Most hernias were in
midline incisions in a population in which the transverse incision
was the primary approach at cesarean delivery. Consequently the
risk of developing a hernia in a transverse cesarean incision
following current surgical recommendations is very low and should
not restrict the use of cesarean delivery.
We thank Steen Rasmussen, University of Copenhagen, Hvidovre Hospital
for his assistance with data management. He was paid for the service
through project funds.
Edwin Stanton Spencer edited the language in the article and was paid
for the service through project funds.
Conceived and designed the experiments: AA LK SL NJS. Performed the
experiments: AA LK SL. Analyzed the data: AA SL. Contributed
reagents/materials/analysis tools: SL AA. Contributed to the writing of
the manuscript: AA.
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