Definition of Preterm Labor
Preterm labor is defined as labor occurring before 37 completed weeks of gestation (specifically before 37 0/7 weeks), with the lower boundary beginning at 20 0/7 weeks of gestation. 1, 2
Clinical Diagnostic Criteria
The diagnosis of preterm labor requires both of the following components:
- Regular uterine contractions accompanied by cervical change (dilation and/or effacement) 3, 4
- Occurrence before 37 0/7 weeks of gestation 1, 2
Gestational Age Boundaries
Lower Boundary
- 20 0/7 weeks marks the threshold of potential viability and serves as the lower boundary for defining preterm labor 2
Upper Boundary
- 36 6/7 weeks represents the upper limit, with term pregnancy beginning at 37 0/7 weeks 2
Clinical Subcategories
Preterm labor is further stratified by gestational age, which has critical implications for management:
Late Preterm Period
- 34 0/7 to 36 6/7 weeks of gestation 1, 2
- Accounts for approximately three-fourths of all preterm births 2
- Characterized by significantly better outcomes than earlier preterm births, though still associated with increased respiratory and metabolic complications compared to term infants 2
Periviable Period
- 20 0/7 to 25 6/7 weeks of gestation 1, 2
- Represents the most critical gestational ages with highly variable survival rates 1
- Survival rates: 5-6% at deliveries before 23 weeks, 23-27% at 23 weeks, 42-59% at 24 weeks, and 67-76% at 25 weeks 1, 5
Previable Period
- Denotes the period when a fetus would not survive outside the uterus and is not a candidate for life-sustaining interventions 1
Important Clinical Caveats
Less than 10% of women with a clinical diagnosis of preterm labor will actually deliver within seven days of initial presentation, highlighting the challenge of accurate diagnosis 3. This underscores why additional diagnostic tools beyond clinical criteria—such as transvaginal ultrasound cervical length measurement and biomarkers like fetal fibronectin—are often employed in clinical practice 6.
The definition encompasses both spontaneous preterm labor (with intact membranes and cervical changes) and preterm labor associated with preterm premature rupture of membranes, but excludes indicated preterm deliveries for maternal or fetal conditions 7.