Trial design. This randomized controlled trial was conducted at the Obstetric Unit, Department of Women´s and Children´s Health, Karolinska University Hospital and Karolinska Institutet, Stockholm, Sweden between December 2009 and January 2018.
and registration. Ethics approval was obtained from the Regional Ethics Board for Medical Sciences in Stockholm in September 2007, No 2007-311-31. The trial was registered at the European Union Drug Regulating Authorities Clinical Trials (EudraCT), that participates in the World Health Organization (WHO) International Clinical Trial Registry Platform in September 2007, No 2007-003348-31, and was approved by the Swedish Medical Products Agency in May 2008, No 151:2008/30388. All treatments were performed according to the relevant clinical guidelines and regulations, and all participants were included after informed oral and written consent. Ethics approval for the control group of women, who received standard intravenous tocolytics alone in the same years 2009–18 was obtained from the Regional Ethics Board for Medical Sciences in Stockholm in April 2015, No 2014/255 − 31. Since data from the control group were collected in retrospect and presented on a group basis only, individual consent was not required.
Participant recruitment. Inclusion criteria were singleton pregnancy, intact fetal membranes, and spontaneous PTL between 24 and 28 gestational weeks resulting in a CL < 25 mm determined by transvaginal ultrasound. Exclusion critera were multiple pregnancy, ruptured fetal membranes, cervical dilatation, cervical cerclage, signs or symptoms of chorioamnionitis, previous uterine surgery, prophylactic progesterone treatment, intercurrent maternal diseases, pregnancy complications such as preeclampsia or gestational diabetes, intrauterine fetal growth restriction or fetal malformations (Fig. 1). The participants were informed about the randomized trial by an obstetrician at the hospital, and they received standard intravenous tocolysis before start of the treatments. Randomization was carried out after oral and written consent, using a computerised blocked system and with closed envelopes. Data were collected in retsrospect from the control group of women with identical inclusion and exclusion criteria who received standard intravenous tocolytics alone in the same years 2009–18. Preterm labor was uterine contractions ≥ 2/10 min for > 30 min according to cardiotocography (CTG) recorded in electronic obstetric records (Obstetrix, Cerner AB, Stockholm, Sweden), which resulted in a CL < 25 mm determined by transvaginal ultrasound carried out by a specialist in obstetrics and gynecology due to standardized criteria.
Interventions. The maintenance treatments were daily doses of vaginal progesterone gel (Crinone, 90 mg/dose, Merck KGaA, Germany) or daily doses of the placebo gel, which is an emulsion of oil and water with an acidic pH 3.0 (Replens, CampusPharma AB, Sweden). It was not possible to blind the gel packages due to practical reasons. The treatments started after standard intravenous tocolytics and continued until 34 + 0 weeks, rupture of the fetal membranes or childbirth whatever occurred first. In all years studied, standard intravenous tocolytics consisted of a bolus dose of the oxytocin receptor antagonist atosiban (Tractocile, Ferring Pharmaceuticals, Sweden) 6.75 mg followed by infusion of 300 µg/min during 3 hours and thereafter 100 µg/min until 48 hours. Alternatively, a β2-adrenergic receptor agonist terbutaline (Bricanyl, AstraZeneca PLC, Great Britain) 5 µg/mL was given for 48 hours according to the individual obstetrician´s choice. All women received two doses of betamethasone (Betapred, Swedish Orphan Biovitrum AB) 12 mg intramuscularly 12 ─ 24 hours apart for fetal lung maturation. In line with standard clinical routines, the participants with PTL and intact fetal membranes did not receive prophylactic antibiotics. All women in active labor at a gestational age < 37 + 0 weeks received intrapartum prophylaxis with bensylpenicillin 3 g every 6 hours. All women in all groups were seen at least weekly by an obstetrician after discharge from the antenatal unit in case of regression of PTL.
Outcomes. The analyses included all randomized participants according to the intention to treat concept. We did not epect drop outs, since the participants were randomized after thorough informed consent and were seen frequently by an obstetrician. The primary outcome latency to delivery was calculated from the first gel dose to childbirth in the treatment groups, and from the start of intravenous tocolysis to childbirth in the control group. The secondary outcomes were delivery ≤ 7 days, rates of PTB < 34 weeks and < 37 weeks, neonatal birth weight (BW), composite neonatal morbidity, NICU admission and length of NICU stay. Composite neonatal morbidity was Apgar score < 7 at 5 min, the incidence of neonatal respiratory distress syndrome (RDS), intraventricular hemorrage (IVH), necrotizing enterocolitis (NEC) and sepsis ≤ 7 days, taken together with retinopathy of prematurity (ROP) and neonatal death during NICU stay. Neonatal RDS was defined by clinical diagnosis of type I RDS and a requirement of oxygen therapy for at least 24 hours. Maternal adverse effects such as fatigue, headache, or intrahepatic cholestasis were monitored.
Sample size. We expected, that the latency to delivery would be 30 % longer after progesterone compared to placebo. According to a power analysis, a sample size of n = 29 in each group would be required to reach a significance of 5 % and power of 80 % .
Statistical analysis. Continuous data were analyzed using Mann Whitney U-test, and were presented as mean ± standard deviation (SD). Categorical data were analyzed with Chi2-test and Fisher´s exact test when appropriate, and were presented as numbers and percentages. Confidence intervals and composite neonatal morbidity were analyzed with One way ANOVA. A two-tailed p value < 0.05 was considered significant.