Prediction and Prevention of Preterm Birth

SUMMARY:

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
    • Progesterone (vaginal or IM) starting at 16w0d to 20w6d “more effective than starting later” | “Although the pharmacokinetic data are reassuring, there are no clinical data on which to make a recommendation regarding use of subcutaneous 17-OHPC in pregnant individuals at risk for preterm birth”
    • Serial endovaginal cervical length measurements starting at 16w0d and repeated every 1 to 4 weeks until 24w0d
    • If cervical length ≤25 mm, offer the following
      • Not on progesterone: Cervical cerclage or vaginal progesterone
      • On progesterone: Cervical cerclage and continue progesterone

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
    • Vaginal Progesterone: “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)
      • Exam-indicated: 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

  • Progesterone: No reduction of PTB
  • Cerclage if cervix <25 mm: May increase risk for PTB
  • “There is a paucity of information about optimal cervical length screening and treatment approaches for individuals with higher-order multiple gestations”
  • 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”

KEY POINTS:

  • Women with singleton pregnancy and history of PTB should be offered progesterone supplementation staring at 16 weeks
  • 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

Learn More – Primary Sources:

ACOG Practice Bulletin 234: Prediction and Prevention of Preterm Birth

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

CDC: Premature Birth