Fig. 1

Study sample selection. Notes:¹ High risk pregnancy defined by the presence of one or more risk factors among: multiple pregnancy, non-cephalic presentations, BMI > 27.5, previous CS, hypertensive disorders (pregestational hypertension, preeclampsia, eclampsia, HELLP syndrome), chorioamnionitis, foetal malformations, IUGR/SGA, pregestational or gestational diabetes in drug therapy, maternal cardiac disease, polyhydramnios, oligohydramnios, APH, severe anaemia, systemic lupus erythematosus, pre-pregnancy deep venous thrombosis, epilepsy, pelvic dysfunction, recurrent infection, pancreatitis or glomerulonephritis in pregnancy, chickenpox disease, chronic disease, signs of potentially impaired foetal wellbeing (i.e. non-reassuring or pathological cardiotocography, reduced foetal movement, meconium stained amniotic fluid). We also excluded macerated stillbirth from the group IOL at 40 GW, as these births are always induced. ² Reported indications for IOL suggesting the presence of the above risk factors. Abbreviations: APH = Antepartum haemorrhage; BMI = Body mass index; CS = Caesarean section; GW = gestational weeks; IOL = induction of labour; IUGR = Intrauterine growth restriction at ultrasonography; SGA = Small for gestational age; SOL = spontaneous onset of labour