|dc.description.abstract||Aims: Obstetric anal sphincter injuries (OASIS) are serious complications of vaginal
childbirth and may result in severe consequences such as dyspareunia, anal- and
urinary incontinence. In Norway and other Scandinavian countries, the reported
occurrence of these injuries has increased during the past decades. The aims of this
thesis were first to validate the registration of OASIS in two Norwegian databases, the
Medical Birth Registry of Norway (MBRN) and Patient Administration System (PAS).
Secondly, we wanted to investigate risk factors and secular trends of OASIS in
Norway in 1967-2004 and whether changes in risk factors over time could explain the
trends. Thirdly, we wanted to study the obstetric history of a woman with OASIS in
terms of recurrence risk, likelihood of having a subsequent delivery and mode of
delivery. Finally, we wanted to assess possible familial aggregation of OASIS among
Methods: All four papers are historic cohort studies. In paper I, data on OASIS cases
occurring at Haukeland University Hospital during 1990-92 and 2000-02 were derived
from PAS and MBRN. The registration of OASIS was validated by comparing these
two registries with patient hospital records as the gold standard. Papers II-IV were
population-based studies based on data from MBRN 1967-2008. We used contingency
tables, logistic regression, Cox proportional hazards regression and stratification to
explore associations between various exposures and outcomes, to assess interactions
and to adjust for confounders.
Results: The sensitivity and specificity of the MBRN database to detect OASIS were
85.3% and 99.5% in 1990–92, 91.8% and 99.7% in 2000–02, respectively. The
positive and negative predictive values of OASIS in the MBRN were 91.4% and
99.1% in 1990-92 and 95.4% and 99.4% in 2000–02. The sensitivity and specificity of
the PAS database were correspondingly 52.1% and 99.0% in 1990–92 and 84.6% and
98.5% in 2000–02. The positive and negative predictive values of OASIS in PAS
database were 75.8% and 97.1% in 1990–92 and 92.7% and 98.9% in 2000-02. The reported occurrence of OASIS increased from 0.5% in 1967 to 4.1% in 2004.
After adjustment for changes in demographic and other risk factors, the increase of
OASIS persisted, although significantly reduced. OASIS were associated with
maternal age 30 years or more, vaginal birth order 1, previous caesarean delivery,
instrumental delivery, diabetes type 1, gestational diabetes, induction of labour by
prostaglandin, large maternity units, birth weight 3,500 g or more, head circumference
35 cm or more and African or Asian women’s country of birth. Only in birth order 1
with instrumental delivery, episiotomy seemed to protect perineum against OASIS;
otherwise it either increased the risk of OASIS or gave no protection against OASIS.
Women with a history of OASIS in the first and the two first deliveries had four and
ten fold increased risk of OASIS in the subsequent delivery, respectively. Populationattributable
risk percentage of OASIS in second and third delivery due to previous
OASIS was 10% and 15%, respectively. Recurrence of OASIS was high in large
maternity units, in forceps delivery and with birth weight 3,500 g or more in the
current delivery. However, instrumental delivery did not further increase the excess
recurrence risk observed in heavy newborns.
A man who fathered a child whose delivery was complicated by OASIS in one woman
was more likely to father another child with OASIS delivery in another woman, if the
mothers delivered at the same maternity unit.
The subsequent delivery rate was not different in women with and without previous
OASIS, whereas women with previous OASIS were more often scheduled to
The risk of OASIS was increased two fold if a woman’s mother or sister had sustained
OASIS and to a less extent if her partner’s mother or sister had sustained OASIS, and
not if her brother’s partner had sustained OASIS.
Conclusions: The validity of the registration of OASIS in MBRN is sufficiently high
to justify epidemiological studies on OASIS based on data from this registry. The risk
of OASIS increased noticeably in 1967-2004 in Norway. Changes in observed risk factors could only partially explain this increase. Most of observed risk factors such as
birth order 1 and high maternal age were non-modifiable and women with such risk
factors should be paid more attention at delivery for minimising their risk of OASIS.
Instrumental delivery was a dominant risk factor, but the majority of OASIS cases
occurred in non-instrumental vaginal deliveries. Consequently, training in both
instrumental and non-instrumental deliveries with focus on reducing the speed of the
birth, support of perineum and axis of birth canal should be an essential part of the
national and local training programme for birth attendants.
Women with a history of OASIS had a high recurrence risk in second and third
delivery. Therefore, emphasis should be placed on counselling women after an initial
OASIS. A history of OASIS had little or no impact on subsequent delivery rate.
However, women with previous OASIS more frequently had planned caesarean
Our findings in paper IV suggest that maternal and to a less extent paternal factors
contribute to the risk of OASIS. The higher maternal than paternal recurrence of
OASIS indicate maternal rather than paternal genetic susceptibility for OASIS. These
observations must be cautiously interpreted since bias due to unmeasured confounding
may have impacted the findings.||en
|dc.publisher||The University of Bergen||eng
|dc.relation.haspart||Paper I: Baghestan E, Børdahl PE, Rasmussen S, Sande AK, Lyslo I, Solvang I. A validation of the diagnosis of obstetric sphincter tears in two Norwegian databases, the Medical Birth Registry and the Patient Administration System. Acta Obstetricia et Gynecologica Scandinavica 86(2): 205-209, 2007. Full text not available in BORA due to publisher restrictions. The article is available at: <a href="http://dx.doi.org/10.1080/00016340601111364" target="blank"> http://dx.doi.org/10.1080/00016340601111364</a>||eng
|dc.relation.haspart||Paper II: Baghestan E, Irgens LM, Børdahl PE, Rasmussen S. Trends in risk factors for obstetric anal sphincter injuries in Norway. Obstetrics & Gynecology 116(1): 25-34, July 2010. Full text not available in BORA due to publisher restrictions. The article is available at: <a href="http://dx.doi.org/10.1097/AOG.0b013e3181e2f50b" target="blank"> http://dx.doi.org/10.1097/AOG.0b013e3181e2f50b</a>||eng
|dc.relation.haspart||Paper III: Baghestan E, Irgens LM, Børdahl PE, Rasmussen S. Risk of recurrence and subsequent delivery after obstetric anal sphincter injuries. BJOG: An International Journal of Obstetrics & Gynaecolog 119(1): 62–69, January 2012. Full text not available in BORA due to publisher restrictions. The article is available at: <a href="http://dx.doi.org/10.1111/j.1471-0528.2011.03150.x" target="blank"> http://dx.doi.org/10.1111/j.1471-0528.2011.03150.x</a>||eng
|dc.relation.haspart||Paper IV: Baghestan E, Irgens LM, Bordahl PE, Rasmussen S. Familial risk of obstetric anal sphincter injuries: registry-based cohort study. Full text not available in BORA.||eng
|dc.title||Epidemiological aspects of Obstetric Anal Sphincter Injuries. A population-based study in Norway||eng
|dc.subject.nsi||VDP::Medical disciplines: 700::Clinical medical disciplines: 750::Gynecology and obstetrics: 756||eng
|dc.rights.holder||Copyright the author. All rights reserved||