What is the most commonly used measure of true preterm labor, i.e., duration from evaluation to actual delivery?

Cervical Length Measurement is the Most Commonly Used Measure of True Preterm Labor

Transvaginal ultrasound measurement of cervical length (CL) is the most commonly used and reliable measure to predict true preterm labor, with delivery within 7 days of presentation being the standard timeframe used to define true preterm labor. 1

Diagnostic Criteria and Predictive Value

Transvaginal ultrasound (TVUS) is considered the reference standard for assessing the gravid cervix due to its ability to completely visualize the cervix, including the internal os where changes associated with preterm birth (PTB) first occur. The measurement provides highly reproducible results that correlate with delivery outcomes:

• Short cervical length strongly predicts imminent delivery:

  • CL ≤15 mm: Associated with 66.7% likelihood of delivery <35 weeks (vs. 13.5% with CL >15 mm) 1
  • CL ≤15 mm: 77% sensitivity and 77% specificity for predicting delivery within 7 days 1
  • CL ≤15 mm: Significantly higher risk of delivery <32 weeks, within 48 hours, and within 7 days of admission 1

• Longer cervical length indicates lower risk:

  • CL ≥30 mm: Very low risk of spontaneous preterm birth, delivery within 48 hours, or within 7 days of admission 1

Timeframes Used to Define True Preterm Labor

The literature consistently uses specific timeframes from evaluation to delivery to define true preterm labor:

1. Delivery within 7 days of presentation - primary measure of true preterm labor 1, 2
2. Delivery within 48 hours of presentation - shorter-term outcome measure 1, 2
3. Delivery before 35 weeks gestation - longer-term outcome measure 1, 2

These timeframes are clinically relevant because they align with important intervention windows:

• The 48-hour window corresponds to the time needed for antenatal corticosteroids to achieve maximum benefit 3
• The 7-day window is commonly used in research and clinical practice to distinguish true from false preterm labor 2

Clinical Application Algorithm

When evaluating a patient with suspected preterm labor:

1. Perform transvaginal ultrasound measurement of cervical length

   • This should be the first-line diagnostic test 1
   • Ensure complete visualization of the cervix, including the internal os

2. Interpret findings based on established thresholds:

   • CL ≤15 mm: High risk of delivery within 7 days (true preterm labor)
   • CL 16-25 mm: Moderate risk
   • CL ≥30 mm: Low risk of delivery within 7 days (likely false preterm labor)

3. Consider additional risk factors that may modify prediction:

   • Presence of vaginal bleeding significantly increases risk of delivery within 7 days 2
   • Cervical funneling is an independent predictor of delivery <37 weeks 1

Limitations and Caveats

Despite its value, cervical length measurement has some limitations:

• The overall accuracy of using CL to predict timing of spontaneous preterm birth is relatively poor, with its greatest value being its high negative predictive value 1
• In patients with cervical dilation ≥3 cm, the predictive accuracy of CL decreases 1
• Transperineal ultrasound may be used as an alternative when TVUS is contraindicated (e.g., placenta previa, suspected preterm premature rupture of membranes), but it typically underestimates cervical length by approximately 1.5 mm 1

Alternative Approaches

While cervical length is the most commonly used measure, other approaches include:

• Fetal fibronectin testing (sometimes used as complementary to CL measurement) 4
• Clinical criteria including regular contractions with cervical change 3

However, these alternatives have not demonstrated the same level of evidence and standardization as cervical length measurement for predicting true preterm labor.