Preterm Labor

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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


Preterm birth is the leading cause of perinatal morbidity and mortality in the United States (1).

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

10% of all births in the United States are preterm, of which 2/3 are spontaneous, and 1/3 are medically indicated.

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., urinary tract infection [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. Numerous gene candidates mediating various pathways (inflammation, apoptosis, coagulation, hypoperfusion, thrombosis, collagen remodeling) have been identified, but causality and gene-environment interactions are not well-defined.

Risk Factors

  • Demographic factors, including single parent, poverty, and black race
  • Short interpregnancy interval (<18 months)
  • 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
  • Routine transvaginal ultrasound cervical length (CL) measurements in singleton pregnancies in the mid-2nd trimester to detect increased risk for preterm birth (<20 mm) is an acceptable strategy (2) and may be especially helpful if risk factors for preterm delivery are present.
  • Primary prevention:
    • Interval contraception to optimize pregnancy spacing
    • Smoking cessation
    • 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 (2)[A] or pessary placement (3)[B] before 24 weeks’ gestation for those at high risk because of cervical insufficiency or significant or progressive cervical shortening.
  • Secondary prevention:
    • 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],(4)[B].
    • Tocolysis

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