Definitions and interventions
OASI was diagnosed according to the International Classification of
Diseases (ICD) 9 definition 664.2 and 664.3 (similar to ICD 10 codes
O70.2 and O70.3). The diagnosis was set at the time of the tear by the
midwife or physician in charge of the delivery and subsequently
confirmed by a specialist in obstetrics and gynecology.
In addition to degree of perineal rupture, modifiable and non-modifiable
variables regarding the infant, mother and birth process were
registered. Non-modifiable variables included birth weight (in grams),
length (in cm), head circumference (in cm), gestational age (in days),
and Apgar score at 1 and 5 minutes postpartum of the newborn, and
maternal age (in years), parity, duration of the first and second stage
of labour (in minutes), and fetal presentation (occiput posterior,
occiput anterior, deep transverse, breech). Modifiable variables
included the mother’s birth position (supine/sitting, side bearing,
standing, kneeling, and on stool), induction of labour (yes/no),
amniotomy (yes/no), episiotomy (mediolateral, yes/no), augmentation with
oxytocin (yes/no), application of fundal pressure (yes/no), and
instrumental delivery by vacuum extraction (yes/no) or forceps (yes/no).
Methods used for induction of labour were based on the Bishop scores and
included membrane sweeping, transcervical Foley catheter, prostaglandin
vaginal tablets, amniotomy or/and augmentation with oxytocin. Amniotomy
was performed in births with a spontaneous onset to shorten duration of
the first and second stage of labour or when continuous surveillance of
the fetus with a scalp-electrode or an examination of the amniotic fluid
was considered necessary. Augmentation with oxytocin was used after
amniotomy in cases of labour dystocia in births with spontaneous onset.
Indications for performing an episiotomy included imminent fetal
asphyxia, preterm birth, and instrumental vaginal delivery which
included vacuum extraction and the use of forceps at the physician’s
discretion.
Birth weight was measured and registered in grams and subsequently
categorized into quartiles: <3300, 3300–3650, 3660–4040, and
≥4040 grams. Crown-heel length and head circumference were measured in
centimeters according to protocol. Gestational age was estimated
according to routine ultrasonography at 18–20 weeks of gestation at
Lillehammer Hospital. Apgar scores were assessed at one and five minutes
following birth by midwives or physicians. The respective scores were
subsequently dichotomized into <7 or ≥7. Maternal age was
registered in years and subsequently categorized into the following
three groups: <25, 25–29, and ≥30 years. The cases and
controls were stratified to primiparous (first birth) or multiparous
(≥second birth).