Exposure and covariates
Baseline characteristics were extracted from the Swedish
Pregnancy Register. Maternal age was categorized into <25,
25-30, 31-35 and >35 years of age at delivery. Parity was
categorized into nulliparous, primiparous or multiparous (≥2 para).
Early pregnancy BMI (based on early pregnancy weight and height measure)
was categorized according to WHO’s nutritional status categories into
underweight (BMI <18.5), normal weight (BMI 18.5-24.9),
overweight (BMI 25.0-29.9) and obesity (BMI ≥30). Information on
educational level, smoking status in early pregnancy, and selected
pre-pregnancy comorbidities was also obtained from the register
(Supportive information eTable 3) according to self-reported information
collected and transferred by the midwife at the first antenatal visit.
Educational level was categorized into less than 9 years of schooling,
at least elementary school (or equivalent) and up to high school (9-12
years of schooling), and higher level of education, i.e. university,
college or equivalent (>12 years of schooling).
Self-reported presence of pre-pregnancy diabetes mellitus, essential
hypertension and recurrent urinary tract infection was reported as
yes/no, as was smoking-status in early pregnancy.
Delivery characteristics were also obtained from the Swedish
Pregnancy Register. Gestational age at delivery was further stratified
into early term (between 37+0 and 38+6/7 gestational weeks), full term
(between 39+0 and 40+6/7 gestational weeks), late term (between 41+0 and
41+6/7 gestational weeks) and post term deliveries (≥ 42+0 gestational
weeks). The delivery onset was stratified into spontaneous, medically
induced and cesarean delivery before onset of contractions. Mode of
delivery was categorized into non-instrumental vaginal delivery,
instrumental delivery (vacuum extraction) and cesarean delivery
according to registration in the Swedish Pregnancy Register.
Specific clinical and laboratory characteristics used to diagnose
chorioamnionitis were collected from medical charts. Information on
maternal temperature during delivery was obtained and reported as: i) no
reported temperature ≥38.0°C, ii) at least one reported temperature
between 38.0°C and 38.9°C, iii) reported temperature between 38.0 and
38.9°C persisting for at least 30 minutes, and iv) at least one reported
temperature of 39.0°C or above. First and highest leucocyte count and
CRP level was extracted from medical charts and categorized into
tertiles. First reported values were, in general, based on blood samples
taken in conjunction with diagnosis (during labor, prior to or in
conjunction with delivery), whereas highest laboratory values were based
on follow-up laboratory values, including postpartum care. Fetal
tachycardia was defined as a fetal heart rate above 160 beats per minute
(bpm) (and a duration of >10 minutes) during a period of
maternal fever, and was extracted from the cardiotocography (CTG)
registration. Maternal tachycardia was defined as heart rate above 100
bpm in the presence of maternal fever, and was extracted from text,
registered clinical parameters or if captured on CTG registration (by
pulse oximetry). Clinical findings and symptoms of chorioamnionitis
(purulent cervical drainage, uterine tenderness, maternal malaise and/or
foul-smelling amniotic fluid or discharge) was registered as present if
specified in the medical chart text. A positive culture, cervical or
blood, was defined as the presence of bacteria confirming the diagnosis
microbiologically. Positive pathological-anatomic diagnosis (PAD) was
defined as the presence of acute inflammatory changes in the amnion or
chorion in pathological analysis of the placenta.