Spontaneous or “idiopathic” preterm delivery accounts for at least 50%  of all preterm deliveries (range 23.2%-64.1%), being more frequent in the population without any established risk factors, where it represents up to 50%-70% of all preterm deliveries according to studied populations. Reported risk factors included personal obstetrical history, social factors and lifestyle. Spontaneous preterm birth is preceded by spontaneous  preterm labour, which cannot be stopped in 70% to 80% of cases.