What are the risk factors for preterm labor?

Updated: May 04, 2021
  • Author: Michael G Ross, MD, MPH; Chief Editor: Carl V Smith, MD  more...
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The exact mechanism(s) of preterm labor is largely unknown but is believed to include decidual hemorrhage, (eg, abruption, mechanical factors such as uterine overdistension from multiple gestation or polyhydramnios), cervical incompetence (eg, trauma, cone biopsy), uterine distortion (eg, müllerian duct abnormalities, fibroid uterus), cervical inflammation (eg, resulting from bacterial vaginosis [BV], trichomonas), maternal inflammation/fever (eg, urinary tract infection), hormonal changes (eg, mediated by maternal or fetal stress), and uteroplacental insufficiency (eg, hypertension, insulin-dependent diabetes, drug abuse, smoking, alcohol consumption). [2, 3]  A genomewide association study that included 43,568 European women identified six genes (BF1, EEFSEC, AGTR2, WNT4, ADCY5, and RAP2C) that were associated with gestational duration, of which, three genes were associated with preterm birth (EBF1, EEFSEC, and AGTR2). [6]

Although prediction of preterm delivery remains inexact, a variety of maternal and obstetric characteristics are known to increase the risk, presumably via one of these mechanisms. Finally, the fetus plays a role in the initiation of labor. In a simplistic sense, the fetus recognizes a hostile intrauterine environment and precipitates labor by premature activation of a fetal-placental parturition pathway.

Risk factors for preterm birth include demographic characteristics, behavioral factors, and aspects of obstetric history such as previous preterm birth. Demographic factors for preterm labor include nonwhite race, extremes of maternal age (< 17 y or >35 y), low socioeconomic status, and low prepregnancy weight. Preterm labor and birth can be associated with stressful life situations (eg, domestic violence; close family death; insecurity over food, home, or partner; work and home environment) either indirectly by associated risk behaviors or directly by mechanisms not completely understood. Many risk factors may manifest in the same gravida.

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