Causes
As the cause of labor still remains elusive, the exact cause of preterm birth is also unsolved. In fact, the cause of 50% of preterm births is never determined. Labor is a complex process involving many factors. Four different pathways have been identified that can result in preterm birth and have considerable evidence: precocious fetal endocrine activation, uterine overdistension (placental abruption), decidual bleeding, and intrauterine inflammation/infection. Activation of one or more of these pathways may happen gradually over weeks, even months. From a practical point a number of factors have been identified that are associated with preterm birth, however, an association does not establish causality.
Read more about this topic: Preterm Birth