What causes preterm labor and birth?

The causes of preterm labor and premature birth are numerous, complex, and only partly understood. Medical, psychosocial, and biological factors may all play a role in preterm labor and birth.

There are three main situations in which preterm labor and premature birth may occur:

• Spontaneous preterm labor and birth. This term refers to unintentional, unplanned delivery before the 37th week of pregnancy. This type of preterm birth can result from a number of causes, such as infection or inflammation, although the cause of spontaneous preterm labor and delivery is usually not known. A history of delivering preterm is one of the strongest predictors for subsequent preterm births.¹
• Medically indicated preterm birth. If a serious medical condition—such as preeclampsia—exists, the health care provider might recommend a preterm delivery. In these cases, health care providers often take steps to keep the baby in the womb as long as possible to allow for additional growth and development, while also monitoring the mother and fetus for health issues. Providers also use additional interventions, such as steroids, to help improve outcomes for the baby.
• Non-medically indicated (elective) preterm delivery. Some late-preterm births result from inducing labor or having a cesarean delivery even though there is not a medical reason to do so, even though this practice is not recommended. Research indicates that even babies born at 37 or 38 weeks of pregnancy are at higher risk for poor health outcomes than are babies born at 39 weeks of pregnancy or later. Therefore, unless there are medical problems, health care providers should wait until at least 39 weeks of pregnancy to induce labor or perform a cesarean delivery to prevent possible health problems.²
  
  The National Child and Maternal Health Education Program, led by NICHD in collaboration with 33 other agencies, organizations, and groups focused on maternal and child health, offers videos and other information about why it’s best to wait until at least 39 weeks of pregnancy to deliver unless there is a medical reason. Learn more about the “Is It Worth It?” initiative.