What are the risk factors for preterm labor and birth?

There are several risk factors for preterm labor and premature birth, including ones that researchers have not yet identified. Some of these risk factors are "modifiable," meaning they can be changed to help reduce the risk. Other factors cannot be changed.

Health care providers consider the following factors to put women at high risk for preterm labor or birth:

• Women who have delivered preterm before, or who have experienced preterm labor before, are considered to be at high risk for preterm labor and birth.¹
• Being pregnant with twins, triplets, or more (called "multiple gestations") or the use of assisted reproductive technology is associated with a higher risk of preterm labor and birth. One study showed that more than 50% of twin births occurred preterm, compared with only 10% of births of single infants.²
• Women with certain anomalies of the reproductive organs are at greater risk for preterm labor and birth than are women who do not have these anomalies. For instance, women who have a short cervix (the lower part of the uterus) or whose cervix shortens in the second trimester (fourth through sixth months) of pregnancy instead of the third trimester are at high risk for preterm delivery.

Certain medical conditions, including some that occur only during pregnancy, also place a woman at higher risk for preterm labor and delivery. Some of these conditions include³:

• Urinary tract infections
• Sexually transmitted infections
• Certain vaginal infections, such as bacterial vaginosis and trichomoniasis
• High blood pressure
• Bleeding from the vagina
• Certain developmental anomalies in the fetus
• Pregnancy resulting from in vitro fertilization
• Having underweight or obesity before pregnancy
• Short time period between pregnancies (less than 6 months between a birth and the beginning of the next pregnancy)
• Placenta previa, a condition in which the placenta grows in the lowest part of the uterus and covers all or part of the opening to the cervix
• Being at risk for rupture of the uterus (when the wall of the uterus rips open). Rupture of the uterus is more likely if you have had a prior cesarean delivery or have had a uterine fibroid removed.
• Diabetes (high blood sugar) and gestational diabetes (which occurs only during pregnancy)
• Blood clotting problems

Other factors that may increase risk for preterm labor and premature birth include:

• Ethnicity. Preterm labor and birth occur more often among certain racial and ethnic groups. For example, infants of African American mothers are more likely to be born preterm than infants of white mothers. American Indian/Alaska Native mothers are also more likely to give birth preterm than are white mothers.⁴
• Age of the mother.
  • Women younger than age 18 are more likely to have a preterm delivery.
  • Women older than age 35 are also at risk of having preterm infants because they are more likely to have other conditions (such as high blood pressure and diabetes) that can cause complications requiring preterm delivery.⁴
• Certain lifestyle and environmental factors, including:³
  • Late or no health care during pregnancy
  • Smoking
  • Drinking alcohol
  • Using illegal drugs
  • Domestic violence, including physical, sexual, or emotional abuse
  • Lack of social support
  • Stress
  • Long working hours with long periods of standing
  • Exposure to certain environmental pollutants