Definition of Preterm Labour
Preterm labour is defined as regular uterine contractions accompanied by cervical change (dilation and/or effacement) occurring between 20 0/7 and 36 6/7 weeks of gestation. 1, 2
Core Diagnostic Criteria
The diagnosis requires both of the following components:
- Regular uterine contractions occurring before 37 0/7 weeks of gestation 1, 2
- Documented cervical change (progressive dilation and/or effacement) 1, 3
Gestational Age Boundaries
The temporal definition has specific upper and lower limits:
- Lower boundary: 20 0/7 weeks of gestation, which marks the threshold of potential viability 1
- Upper boundary: 36 6/7 weeks of gestation, with term pregnancy beginning at 37 0/7 weeks 1, 2
Clinical Subcategories by Gestational Age
Preterm labour is further stratified based on timing, which has significant implications for morbidity and mortality:
Late Preterm Period (34 0/7 to 36 6/7 weeks)
- Accounts for approximately two-thirds to three-fourths of all preterm births 4, 1
- Associated with better outcomes than earlier preterm births but still carries increased respiratory and metabolic complications compared to term infants 1
Early Preterm Period (<34 weeks)
- Represents nearly one-third of preterm births 4
- Associated with significantly higher perinatal morbidity and mortality 4
Periviable Period (20 0/7 to 25 6/7 weeks)
- Characterized by highly variable survival rates: 5-6% before 23 weeks, 23-27% at 23 weeks, 42-59% at 24 weeks, and 67-76% at 25 weeks 1
- Requires individualized counseling about maternal risks and fetal outcomes 2
Previable Period
- Denotes the period when a fetus would not survive outside the uterus and is not a candidate for life-sustaining interventions 1, 5
Important Clinical Caveats
Diagnostic accuracy limitation: Less than 10% of women with a clinical diagnosis of preterm labour will actually deliver within seven days of initial presentation 6. This highlights that the presence of contractions alone, even with some cervical change, does not necessarily predict imminent delivery.
Cervical assessment: A short transvaginal cervical length (<25 mm before 24 weeks' gestational age) is associated with increased risk for preterm delivery, though it represents a risk factor rather than a diagnostic criterion for active preterm labour 4