In nearly 40 percent of premature births, the cause is unknown. However, researchers have made some progress in learning the causes of prematurity. Studies suggest that there may be four main routes leading to spontaneous premature labor.

1. Infections/Inflammation. Studies suggest that premature labor is often triggered by the body's natural immune response to certain bacterial infections, such as those involving the genital and urinary tracts and fetal membranes. Even infections far away from the reproductive organs, such as periodontal disease, may contribute to premature delivery.
2. Maternal or fetal stress. Chronic psychosocial stress in the mother or physical stress (such as insufficient blood flow from the placenta) in the fetus appears to result in production of a stress-related hormone called corticotropin-releasing hormone (CRH).  CRH may stimulate production of a cascade of other hormones that trigger uterine contractions and premature delivery. 
3. Bleeding.  The uterus may bleed because of problems such as placental abruption (the placenta peels away, partially or almost completely, from the uterine wall before delivery). Bleeding triggers the release of various proteins involved in blood clotting, which also appear to stimulate uterine contractions.
4. Stretching.  The uterus may become overstretched by the presence of two or more babies, excessive amounts of amniotic fluid, or uterine or placental abnormalities, leading to release of chemicals that stimulate uterine contractions.

These four routes are not the only things to consider. Other factors, such as multiple pregnancy, inductions and cesarean sections, can also play a role. But knowledge about these four routes may help scientists develop more effective interventions that can halt the various chemical cascades that lead to premature birth. 

July 2009