Cause

Preterm labor can be caused by a problem involving the fetus, the mother, or both. Often a combination of several factors is responsible. But in about 1 out of 3 preterm births, the cause is not known.²

Causes of spontaneous preterm labor include:

• 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.