Being born before 37 weeks gestational age or before 259 days, is defined as preterm birth according to the World Health Organisation.  In this definition, the lower limit is not specified. Preterm birth could be qualified according to birthweight with large variations depending on the studied populations or to gestational ages strata. Preterm birth could also be  categorised by its clinical presentations: medically induced, preterm premature rupture of membranes and spontaneous preterm labour leading to preterm delivery. Several aetiologies and/or risk factors have been reported for each of the three categories although none completely explain all preterm births. Recent investigations, more directed to defined plausible biological pathways, may reconcile the apparent heterogeneity of preterm birth