Preterm labor can be caused by a problem involving the fetus, the mother, or both. Often a combination of several factors is responsible. In about 30% of cases, the cause of preterm labor is not known.²

Causes of spontaneous preterm labor include:

• Multiple pregnancy, accounting for about 30% of preterm births.² Women who are pregnant with more than one fetus have an increased risk of complications-both maternal and fetal-and typically deliver early. See the topic Multiple Pregnancy: Twins or More for more information.
• Infection, which can trigger uterine contractions and preterm premature rupture of membranes (pPROM).
• Placenta abruptio, the early separation of the placenta from the uterus. For more information, see the topic Placenta Abruptio.
• The use of drugs such as cocaine or methamphetamine.
• Problems with the uterus or cervix, such as a weak, thin cervix; fibroid growth; or an abnormally shaped uterus.

Some pregnancy complications endanger the mother or the fetus, making it necessary to deliver a baby early. This is called an indicated preterm birth, accounting for 15% to 20% of all preterm births.² Whenever possible, medication is used to start (induce) a vaginal birth in these cases. However, some conditions require a cesarean delivery. Reasons for indicated preterm birth include some cases of:

• Preeclampsia, which can affect the mother's liver, kidneys, placenta, and brain. This causes high blood pressure and swelling and sometimes poor fetal blood supply or maternal seizures (eclampsia). For more information, see the topic Preeclampsia and High Blood Pressure During Pregnancy.
• Poor fetal growth, known as intrauterine growth restriction (IUGR).
• Placenta previa, when the placenta covers some or all of the opening to the birth canal. This is dangerous for both the mother and her baby because a vaginal birth would cause severe bleeding. To prevent this, a cesarean delivery is done before contractions start, or if there is heavy bleeding. Nearly half of placenta previa deliveries are preterm because of bleeding.³ For more information, see the topic Placenta Previa.