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.