Preterm birth risk factors: who is more likely

What preterm birth is

A normal pregnancy is about 40 weeks long. A birth before 37 weeks of pregnancy is a preterm or premature birth, and labor that begins before 37 weeks is preterm labor. "Late preterm" means 34 through 36 weeks; more than 70% of preterm infants are born in that window (NICHD). MedlinePlus says almost 1 of every 10 infants born in the United States is premature (MedlinePlus).

NICHD calls preterm birth the most common cause of infant death and the leading cause of long-term disability in children. The earlier the delivery, the higher the risk of serious disability or death. Infants born early are at risk for cerebral palsy, developmental delays, and vision and hearing problems (NICHD). MedlinePlus adds breathing problems and feeding difficulties, and says preemies need special care in a neonatal intensive care unit (NICU) (MedlinePlus).

Why it happens

NICHD says the causes are numerous, complex and only partly understood; medical, psychosocial and biological factors may all play a role (NICHD). It describes three main situations:

• Spontaneous preterm labor and birth, an unplanned delivery before 37 weeks. It can result from causes such as infection or inflammation, but the cause is usually not known.
• Medically indicated preterm birth. If a serious condition such as preeclampsia exists, the provider might recommend delivering early.
• Non-medically indicated (elective) early delivery, by induction or cesarean without a medical reason. NICHD says this is not recommended: unless there are medical problems, providers should wait until at least 39 weeks.

Who is more likely to deliver early

NICHD says health care providers consider these to put a woman at high risk (NICHD):

• A past preterm birth or preterm labor. NICHD also calls a history of delivering preterm one of the strongest predictors of later preterm births (NICHD).
• Twins, triplets or more, or the use of assisted reproductive technology. In one study NICHD cites, more than 50% of twin births were preterm, compared with 10% of single births.
• Certain differences in the reproductive organs, such as a short cervix, or a cervix that shortens in the second trimester instead of the third.

Medical conditions that raise the risk, including some that occur only in pregnancy (NICHD):

• Urinary tract infections, sexually transmitted infections, and some vaginal infections such as bacterial vaginosis and trichomoniasis
• High blood pressure (see blood pressure screening in pregnancy)
• Bleeding from the vagina
• Certain developmental differences in the fetus
• Pregnancy from in vitro fertilization
• Being underweight or having obesity before pregnancy
• Less than 6 months between a birth and the start of the next pregnancy
• Placenta previa, when the placenta covers all or part of the opening to the cervix
• Being at risk for rupture of the uterus, which is more likely after a prior cesarean delivery or removal of a uterine fibroid
• Diabetes and gestational diabetes
• Blood clotting problems

Other factors NICHD lists (NICHD):

• Race and ethnicity. Infants of African American mothers and of American Indian/Alaska Native mothers are more likely to be born preterm than infants of white mothers.
• Age. Women younger than 18 are more likely to deliver preterm. Women older than 35 are also at risk, because they are more likely to have conditions such as high blood pressure and diabetes that can lead to an early delivery.
• Lifestyle and environment: late or no health care during pregnancy, smoking, drinking alcohol, using illegal drugs, domestic violence (physical, sexual or emotional abuse), lack of social support, stress, long working hours with long periods of standing, and exposure to certain environmental pollutants.

NICHD notes that some of these can be changed to help lower the risk and others cannot, and that some risk factors have not yet been identified. It gives no score (NICHD).

Signs of preterm labor

Preterm labor is labor from 20 weeks through 36 weeks of pregnancy. NICHD lists these symptoms (NICHD):

• Contractions (tightening of the stomach muscles) every 10 minutes or more often
• A change in vaginal discharge, such as leaking fluid or bleeding
• A feeling of pressure in the pelvis (hip) area
• A low, dull backache
• Cramps that feel like menstrual cramps
• Abdominal cramps, with or without diarrhea

Some contractions during the day are normal. Frequent contractions, such as six or more in one hour, are not, and may cause the cervix to begin to open (NICHD).

Related pages

Preeclampsia is a separate condition that can lead to a medically indicated early delivery; see preeclampsia risk factors and the preeclampsia aspirin check. For bacterial vaginosis testing in pregnancy, see the BV screening check. For pregnancy loss before 20 weeks, see miscarriage risk factors. NICHD lists preterm birth among the risk factors for sudden infant death syndrome; see SIDS risk factors (NICHD).

Common questions

What is the biggest risk factor for preterm birth?

NICHD calls a history of delivering preterm one of the strongest predictors of later preterm births (NICHD).

Is 37 weeks preterm?

No. Births before 37 weeks are preterm. Births in weeks 37 and 38 are called "early term", and NICHD says those babies face more health risks than babies born at 39 or 40 weeks (NICHD).

Can preterm birth be predicted?

Not fully. NICHD says the cause of spontaneous preterm labor is usually not known (NICHD). Ask your provider about your own risk factors.