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Prediction and Prevention of Preterm Birth


Spontaneous preterm birth (PTB) includes preterm labor, preterm rupture of membranes and cervical insufficiency from ≥20w7d to <37w0d of gestation. The PTB rate has significantly increased during the last 2 decades. According to the CDC, PTB rates decreased from 2007 to 2014, partly due to fewer teens and young women giving birth. However, the PTB rate rose for the fifth year in a row in 2019 and sits at approximately 10%. Unfortunately, racial, and ethnic differences in PTB rates remain problematic. The CDC reports that “in 2019, the rate of preterm birth among non-Hispanic black women (14.4%) was about 50% higher than the rate of preterm birth among non-Hispanic white women (9.3%) or Hispanic women (10%).”

Adverse Outcomes and Preterm Birth

  • The rate of neonatal adverse outcomes decreases with advancing gestational age
    • While adverse outcomes are therefore greatest <34 weeks (early preterm), higher rates of both short- and long-term complications are seen between 34w0d an 36w6d (late preterm) vs ≥37 weeks
  • The CDC reported that in 2018, preterm birth and low birth weight accounted for about 17% of infant deaths <1 year
  • Associated adverse outcomes include
    • Respiratory complications | Feeding difficulties | Cerebral palsy | Developmental delay | Vision and hearing impairment

Risk Factors 

  • History of prior PTB
    • Number of prior preterm birth deliveries
    • Gestational age at delivery
  • Short cervical length (<25 mm) between 16 and 24 weeks
  • Behavioral factors
    • Low maternal pre-pregnancy weight (BMI <18.5)
    • Smoking
    • Substance abuse
    • Short interpregnancy interval (<18 months)

Note: Surgical procedures (e.g., cold-knife conization, loop electrosurgical excision, or laser ablation) have been postulated to be associated with preterm birth, but data is inconsistent

Clinical Evaluation and Management

Previous Spontaneous PTB (Singleton)

  • Risk assessment
    • Detailed medical history and prior obstetric history
  • Management
    • Insufficient data to recommend IM 17-OHPC
    • Serial endovaginal cervical length measurements starting at 16w0d and repeated every 1 to 4 weeks until 24w0d
    • If cervical length ≤25 mm, consider the following
      • Vaginal progesterone (vs cerclage)
      • Cervical cerclage (vs vaginal progesterone if not already on supplementation)
      • Physical exam indicated cerclage
    • Cervical pessary: Not indicated

No Previous History of PTB

  • Low risk for PTB
    • Clinical utility of universal cervical length screening “remains unsettled”
    • Cervix should be visualized at the 2nd trimester anatomy exam (18 to 22 weeks) | Transabdominal or endovaginal approach is acceptable
    • If cervix appears short on transabdominal scan, endovaginal ultrasound is recommended to determine whether progesterone may be of benefit
    • Serial endovaginal ultrasonography is not indicated in low risk patients
  • Short cervical length (≤25 mm)
    • IM 17-OHPC: Not indicated
    • Vaginal Progesterone: Indicated | “Although most studies used 200 mg progesterone daily from the time of identification of a cervix shorter than 25 mm at 18 0/7–25 6/7 weeks of gestation until 36–37 weeks of gestation, there are no adequate dosing studies or comparative trials, and there are insufficient data to indicate which formulation and which dose are most effective”
    • Cervical cerclage
      • Ultrasound-indicated: Overall, no significant reduction of PTB | May be potential benefit in very short cervix (<10 mm)
      • Physical exam-indicated: Consider if dilated cervix on digital/speculum exam at 16w0d to 23w6d “are candidates” for cerclage | Uncertain if amniocentesis to test for infection impacts outcome
    • Pessary: Not recommended

Multiple gestation with or without history of PTB

  • Cervix should be visualized at the 2nd trimester anatomy exam (18 to 22 weeks)
  • IM 17-OHPC: Not indicated
  • Vaginal progesterone: Insufficient data
  • Cerclage if cervix ≤25 mm
    • Ultrasound: Insufficient data
    • Physical exam-indicated: Consider procedure
  • Pessary is not recommended

History of a Medically Indicated Preterm Delivery

  • May be increased risk for PTB
  • “insufficient evidence to support a recommendation that these individuals undergo serial cervical length surveillance in future pregnancies”


  • Evidence does not support use of vaginal progesterone to reduce risk of recurrent preterm birth if cervix is not shortened
  • Vaginal progesterone is recommended in women without a history of PTB but with short cervix
  • Screening for fetal fibronectin, bacterial vaginosis and home contraction monitoring are not recommended
  • Universal cervical length with endovaginal remains unclear | However, cervix should be visualized during the second trimester anatomy scan
  • Neither progesterone nor cerclage are recommended routinely in multiple gestations
  • Activity restriction is not recommended to reduce the risk of preterm birth

Note: SMFM addresses vaginal progesterone and recommends shared decision making in the setting of a previous preterm birth but without input of cervical length or cervical length 25 mm or greater | This approach should especially be considered if the progesterone therapy for PTB prevention was used in a prior pregnancy

Learn More – Primary Sources:

ACOG Practice Bulletin 234: Prediction and Prevention of Preterm Birth

ACOG: Updated Clinical Guidance for the Use of Progesterone Supplementation for the Prevention of Recurrent Preterm Birth

SMFM Special Statement: Response to the Food and Drug Administration’s withdrawal of 17-alpha hydroxyprogesterone caproate

AACC Guidance Document on Laboratory Testing for the Assessment of Preterm Delivery

CDC: Premature Birth