What are the main risk factors for preterm birth?

There are two main types of preterm delivery. The first is the type that happens spontaneously, either following preterm labor or a premature of the membranes, and the second is preterm labor that’s induced in response to medical or obstetrical complications endangering the mother or the baby.

Spontaneous preterm delivery

Preterm labor and delivery that happens on its own, instead of being induced by a doctor in the interest of the safety of the mother or baby, doesn’t have one specifically defined cause, but becomes increasingly likely based on a number of risk factors. There are a few risk factors that are considered to put women at high risk for preterm labor and delivery:

  • A short cervix: Women whose cervixes (the narrow passage at the end of the uterus) are shorter than average to begin with, or which shorten in the second trimester instead of the third, are at a high risk for preterm delivery.
  • A history of preterm birth: Women who have experienced premature labor or delivery before are considered much more likely to go into labor prematurely again.
  • Multiple pregnancy: Women who are pregnant with twins, triplets, or other multiples, are much more likely to deliver prematurely. One study suggests that twins may be born preterm as often as 50% of the time.

Of these risk factors, the first two are clear fairly early on. If your healthcare provider thinks you might have a short cervix, he or she may check with a transvaginal ultrasound starting any time between about 16 and 24 weeks of pregnancy. However, most providers do not perform a cervical measurement as a matter of routine, so you may have to specifically request a measurement.

Other risk factors that may increase the risk of preterm labor or delivery include:

  • Becoming pregnant with the help of IVF or other reproductive technologies
  • Being pregnant again within 6 months of a previous pregnancy
  • Having problems with the uterus or placenta
  • Being over- or underweight before pregnancy
  • Not gaining enough weight during pregnancy, or gaining too much weight during pregnancy
  • Emotional distress from stressful life events, or physical stress from physical injury or trauma
  • Some chronic health conditions including high blood pressure and diabetes
  • Some infections, especially infections of the genital tract
  • A fetal birth defect
  • A history of multiple miscarriages or abortions
  • Smoking or drug use

Because some of these risk factors are things that you can control, it’s a great idea to figure out how you can modify your lifestyle to reduce the risk of preterm birth. This could mean monitoring your weight gain throughout pregnancy, going to all of your regular appointments with your provider, quitting smoking or any other kind of substance use, and keeping your body healthy by getting regular exercise (ask your provider about exercise before you start) and eating healthy, nutritious foods.

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  • Mayo Clinic Staff. “Premature birth.” Mayo Clinic. Mayo Clinic, November 27 2014. Web.
  • Mayo Clinic Staff. “Premature labor.” Mayo Clinic. Mayo Clinic, October 19 2016. Web.
  • Julian N. Robinson, Errol R. Norwitz. “Preterm birth: Risk factors and interventions for risk reduction.” UpToDate. Wolters Kluwer, March 7 2017. Web.
  • “Cervical insufficiency and short cervix.” March of Dimes. March of Dimes, August 2015. Web.
  • “Preterm labor and premature birth.” March of Dimes. March of Dimes, March 2016. Web.
  • “What are the risk factors for preterm labor and birth?” Eunice Kennedy Shriver National Institute of Child Health and Human Development. U.S. Department of Health and Human Services, National Institutes of Health. Web.
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