Who is at risk for preterm labor and birth?

Any pregnant woman could have preterm labor and birth, but there are some factors that increase a woman's risk of going into labor before 37 weeks of pregnancy. Some of the risk factors cannot be changed (the woman's race or ethnicity, for example), but some risk factors, such as smoking, can be altered to lower a woman's chances of going into early labor.

Which women are at greatest risk of having a preterm birth?

• One of the most important risk factors for delivering preterm is a history of a previous infant born preterm.
• Being pregnant with more than one fetus at a time (twins or triplets or more) 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.¹
• Women with certain abnormalities of the reproductive organs (uterus and cervix) are at greater risk of going into labor early.

What medical factors affect a woman's risk of preterm labor? 

Figure 1. Cross section of a woman's pelvis with the following organs labeled: fallopian tube, ovary, uterus, cervix, endormetrium, myometrium, vagina.

Certain medical conditions during pregnancy increase the likelihood that a woman will have preterm labor. These conditions include²:

• Having a short cervix in the second trimester (fourth through sixth months) of pregnancy
• Urinary tract infections
• Sexually transmitted infections
• Certain vaginal infections, such as bacterial vaginosis and trichomoniasis
• Diabetes (high blood sugar)
• High blood pressure
• Blood clotting problems
• Bleeding from the vagina
• Certain developmental abnormalities in the fetus
• Pregnancy resulting from in vitro (in a living body) fertilization
• Being underweight before pregnancy
• Obesity (although some studies show that obesity actually protects against preterm birth)
• Short time period between pregnancies (less than 6 months between a birth and the beginning of the next pregnancy)

What other factors affect the risk of giving birth early?

Preterm labor and birth occur more often among certain racial and ethnic groups. Infants of black mothers are 50% more likely to be born preterm than infants of white mothers.³

The age of the mother also affects her risk of having a preterm birth. Women younger than age 18 are more likely to have a preterm delivery. Women over age 35 are 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.³

Some studies have found that certain lifestyle and environmental factors may put a woman at greater risk of preterm labor. These factors include²:

• 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

1. Gardner, M. O., Goldenberg, R. L., Cliver, S. P., Tucker, J. M., Nelson, K. G., & Copper, R. L. (1995). The origin and outcome of preterm twin pregnancies. Obstetrics & Gynecology, 85, 553–557. [top]
2. March of Dimes. (2008, 2010). Preterm labor and birth: A serious pregnancy complication. Retrieved April 23, 2012, from http://www.marchofdimes.com/pregnancy/preterm_indepth.html [top]
3. Centers for Disease Control and Prevention. (n.d.). Preterm birth. Retrieved April 23, 2012, from http://www.cdc.gov/reproductivehealth/maternalinfanthealth/PretermBirth.htm [top]