Every delivery that happens between the 20th and 37th week is considered as a pre-term. The exact mechanism(s) of pre-term labor is largely unknown but is believed to include:
1.decidual hemorrhage - e.g, abruption, mechanical factors such as uterine over-distension from multiple gestation or polyhydramnios;
2.cervical incompetence - (e.g, trauma, cone biopsy;
3.uterine distortion - e.g, müllerian duct abnormalities, fibroid uterus;
4.cervical inflammation - e.g, resulting from bacterial vaginosis, trichomonas;
5.maternal inflammation/fever - e.g, urinary tract infection);
6.hormonal changes - e.g, mediated by maternal or fetal stress), and
7.uteroplacental insufficiency - e.g, hypertension, insulin-dependent diabetes, drug abuse, smoking, alcohol consumption).


Although prediction of pre-term delivery remains inexact, a variety of maternal and obstetric characteristics are known to increase the risk, presumably via one of these mechanisms. Finally, the fetus plays a role in the initiation of labor. In a simplistic sense, the fetus recognizes a hostile intrauterine environment and precipitates labor.


Risk factors for pre-term birth include demographic characteristics, behavioral factors, and aspects of obstetric history such as previous pre-term birth. Demographic factors for pre-term labor include non-white race, extremes of maternal age (<17 y or >35 y), low socio-economic status, and low pre-pregnancy weight. Pre-term labor and birth can be associated with stressful life situations (e.g, domestic violence; close family death; insecurity over food, home, or partner; work and home environment) either indirectly by associated risk behaviors or directly by mechanisms not completely understood.