Preterm Labor

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Basics

Description

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

Epidemiology

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

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

Prevalence
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)

Genetics
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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