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Preterm Labor

Preterm Labor is a topic covered in the 5-Minute Clinical Consult.

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Contractions occurring between 20 and 36 weeks’ gestation at a rate of 4 in 20 minutes or 8 in 1 hour with at least one of the following: cervical change over time or dilation ≥2 cm (1)


Preterm birth is the leading cause of perinatal morbidity and mortality in the United States.

10–15% of pregnancies experienced at least one episode of preterm labor.

~12% of all births in the United States are preterm (9% spontaneous preterm births and 3% indicated preterm births).

Etiology and Pathophysiology

  • Premature formation and activation of myometrial gap junctions
  • Inflammatory mediator–stimulated contractions
  • Weakened cervix (structural defect or extracellular matrix defect)
  • Abnormal placental implantation
  • Systemic inflammation/infections (e.g., UTI, pyelonephritis, pneumonia, sepsis)
  • Local inflammation/infections (intra-amniotic infections from aerobes, anaerobes, Mycoplasma, Ureaplasma)
  • Uterine abnormalities (e.g., cervical insufficiency, leiomyomata, müllerian anomalies, diethylstilbestrol exposure)
  • Overdistension (by multiple gestation or polyhydramnios)
  • Preterm premature rupture of membranes
  • Trauma
  • Placental abruption
  • Immunopathology (e.g., antiphospholipid antibodies)
  • Placental ischemic disease (preeclampsia and fetal growth restriction)

Familial predisposition

Risk Factors

  • Demographic factors, including single parent, poverty, and black race
  • Short interpregnancy interval
  • No prenatal care
  • Prepregnancy weight <45 kg (100 lb), body mass index <20
  • Substance abuse (e.g., cocaine, tobacco)
  • Prior preterm delivery (common)
  • Previous 2nd-trimester dilation and evacuation (D&E)
  • Cervical insufficiency or prior cervical surgery (cone biopsy or loop electrosurgical excision procedure [LEEP])
  • Abdominal surgery/trauma during pregnancy
  • Uterine structural abnormalities such as large fibroids or müllerian abnormalities
  • Serious maternal infections/diseases
  • Bacterial vaginosis
  • Bacteriuria
  • Vaginal bleeding during pregnancy
  • Multiple gestation
  • Select fetal abnormalities
  • Intrauterine growth restriction
  • Placenta previa
  • Premature placental separation (abruption)
  • Polyhydramnios
  • Ehlers-Danlos syndrome

General Prevention

  • Patient education at each visit in 2nd and 3rd trimesters for those at risk and periodically in the last two trimesters for the general population
  • If previous preterm birth, evaluate if etiology is likely to recur and target intervention to specific condition.
    • Weekly injections of 17α-hydroxyprogesterone (250 mg IM every week) from 16 to 36 weeks if previous spontaneous preterm birth
    • Consider cerclage placement before 24 weeks’ gestation for those at high risk because of cervical insufficiency or significant or progressive cervical shortening (2)[A].
  • For women with a short cervix in the 2nd trimester (<20 mm on transvaginal US), progesterone 200 mg/day per vagina for 24 to 34 weeks may decrease the risk of preterm delivery (3)[A].

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Stephens, Mark B., et al., editors. "Preterm Labor." 5-Minute Clinical Consult, 27th ed., Wolters Kluwer, 2019. Medicine Central, im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116485/all/Preterm_Labor.
Preterm Labor. In: Stephens MB, Golding J, Baldor RA, et al, eds. 5-Minute Clinical Consult. 27th ed. Wolters Kluwer; 2019. https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116485/all/Preterm_Labor. Accessed March 22, 2019.
Preterm Labor. (2019). In Stephens, M. B., Golding, J., Baldor, R. A., & Domino, F. J. (Eds.), 5-Minute Clinical Consult. Available from https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116485/all/Preterm_Labor
Preterm Labor [Internet]. In: Stephens MB, Golding J, Baldor RA, Domino FJ, editors. 5-Minute Clinical Consult. Wolters Kluwer; 2019. [cited 2019 March 22]. Available from: https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116485/all/Preterm_Labor.
* Article titles in AMA citation format should be in sentence-case
TY - ELEC T1 - Preterm Labor ID - 116485 ED - Stephens,Mark B, ED - Golding,Jeremy, ED - Baldor,Robert A, ED - Domino,Frank J, BT - 5-Minute Clinical Consult, Updating UR - https://im.unboundmedicine.com/medicine/view/5-Minute-Clinical-Consult/116485/all/Preterm_Labor PB - Wolters Kluwer ET - 27 DB - Medicine Central DP - Unbound Medicine ER -