Factors

There are many different factors which may contribute to a preterm birth.
Factors related to maternal disease or condition that have been shown to increase the risk of preterm birth, with associated odds ratio (OR) when known include:
Chromasomal abnormalities; Dr. Aaron Caughey, a perinatologist at UCSF, states..."...it's important to note that the majority of miscarriages - up to 80 percent - happen due to chromosomal abnormalities that have nothing to do with the mother's behavior. The last thing women who have had miscarriages need to do is blame themselves...." -SF Chronicle
high blood pressure (OR = 4.06)
pre-eclampsia (OR = 4.0)
age > 35 (OR = 1.8)
age < 18 (OR = 3.4)
short cervix (the strongest predictor of premature birth)
maternal diabetes
anxiety
periodontal disease (OR = 4.45%)
Whether or not urinary tract infections directly cause preterm birth is uncertain, however, it is known that urinary tract infections increase pre-eclampsia which as stated above increases the risk of preterm birth. Sexually transmitted disease STD, Beta Strep, kidney disease, and uterine infections are also suspected of increasing the risk of preterm birth.
Adequate maternal nutrition is important to fetaldevelopment and a diet low in saturated fat and cholesterol may help reduce the risk of a preterm delivery.
Factors related to pregnancy history that have been shown to increase the risk of preterm birth include:
prior preterm delivery (OR = 2.79)
prior induced abortion (OR = 1.6)
antepartum hemorrhage / vaginal bleeding during labor
prior miscarriage
Multiple pregnancies (twins, triplets, etc.) are another significant factor in preterm birth. The March of Dimes Multicenter Prematurity and Prevention Study found that 54% of twins were delivered preterm vs. 9.6% of singleton births.

Women who have tried to conceive for more than a year before getting pregnant are at a higher risk for premature birth. A recent study done by Dr. Olga Basso of the University of Aarhus in Denmark and Dr. Donna Baird of the U.S. National Institute of Environmental Health Sciences suggests that women who had difficulty conceiving were about 40 percent higher risk of preterm birth than those who had conceived easily.
Finally, the use of tobacco and alcohol during pregnancy also increases the chance of preterm delivery. Tobacco is the most commonly abused drug during pregnancy and also contributes significantly to low birth weight delivery