What causes premature birth?

No one thing. In America about 12% of women deliver prematurely (before 37 weeks). Sometimes there is an obvious reason like infection, abruption, premature membrane rupture, trauma, history of premature delivery, multiple gestation (twins, triplets), etc. However, the majority of preterm deliveries do not have an obvious cause, and we never know why.
Many possible causes. Premature labour can occur in any pregnancy, not just in ones with known "risk factors". The actual cause or event that starts the contractions is unknown often. If there is an infection, a big uterus stretched by multiple or big babies, bleeding, etc., we might assume that was the cause, but typically it is impossible to prove what actually caused labour. An evaluation after delivery can help.
A spectrum of causes. Spontaneous preterm birth risk factors: maternal race/ethnicity (african-americans have the highest risk), maternal anxiety/depression, social stressors, prior preterm birth, surgery of the cervix, genital infections, bladder infections (the number one cause of preventable preterm birth in usa), multifetal pregnancies (twins etc.), poorly working placenta, genetic predisposition, unknown causes!

Related Questions

Is premature birth one of the main causes of hearing loss?

Association. Conditions associated with premature births are associated with hearing loss. As such, children who have a stay in the neonatal intensive care unit are at greater risk to have hearing loss. This can be associated with jaundice or perinatal infections. Read more...

Is it likely that my infection with legionnaires' disease will cause a premature birth?

See below. According to our infectious disease consultants, the literature on legionnaires disease does not mention a link with premature birth. However, any maternal inflammation or fever increases the risk for preterm labor. Please talk to your ob/gyn soon. Read more...
Legionella may cause PTL. Legionella pneumonia is not uncommon and not easy to diagnose by serology or culture; consequently, there are only a few reports of proven association between the two. Pregnant patients with community acquired pneumonia should be hospitalized and traditional antibiotic therapy should be started promptly without awaiting culture of serology results that can take several days in case of legionella pneumophila infection. Read more...

Is it true that prematurity can cause birth defects upon your child?

PREMATURITY. It depends on several factors. Birth defects are common among small premies when the mother got sick or exposed to rubella for example during the first trimester. Those born with less problems grow up normal. Read more...
Backwards logic. While some babies with birth defects are more likely to be born prematurely, prematurity itself does not cause birth defects. Read more...

Does taking birth control for too long cause extreme prematurity of birth?

No. Although older women (>35 years) who get pregnant for the first time may have increased problems during pregnancy, including preterm birth. Read more...
Nope. Oral contraceptives have nothing to do with prematurity. Stds may lead to prematurity, though, and they are not prevented by ocps. Read more...