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).