Definition of Premature Birth
Premature birth is defined as delivery before 37 completed weeks of gestation (20 0/7 to 36 6/7 weeks). 1
Standard Clinical Definition
- Birth occurring before 37 weeks gestation is universally recognized as preterm across major medical societies and guidelines 1, 2
- The definition begins at 20 0/7 weeks gestation as the lower boundary, which marks the threshold of potential viability 1
- The upper boundary is 36 6/7 weeks, with term pregnancy beginning at 37 0/7 weeks 1, 2
Clinical Subcategories of Preterm Birth
Preterm birth is further stratified by gestational age, which has critical implications for management and outcomes:
Late Preterm (34 0/7 to 36 6/7 weeks)
- Accounts for approximately three-fourths of all preterm births 1
- These infants have significantly better outcomes than earlier preterm births but still face increased respiratory and metabolic complications compared to term infants 1, 3
Moderate Preterm (32 0/7 to 33 6/7 weeks)
- Represents a transitional risk category with substantial morbidity 4
Very Preterm (28 0/7 to 31 6/7 weeks)
- Associated with high rates of respiratory distress syndrome, intraventricular hemorrhage, and long-term neurodevelopmental concerns 4
Extremely Preterm (<28 weeks)
- Carries 30-50% mortality risk and 20-50% morbidity risk in survivors 4
- The periviable period (20 0/7 to 25 6/7 weeks) represents the most critical gestational ages with survival rates ranging from 5-6% at 22 weeks to 67-76% at 25 weeks 5
Global Epidemiology
- Approximately 15 million babies are born preterm annually worldwide, representing about 11% of all births globally 2
- In developed countries, preterm birth rates range from 5-10%, while developing countries may see rates as high as 25% 2, 6
- In the United States specifically, the current preterm birth rate is approximately 11.39% 4
Important Clinical Distinctions
Early Term vs. Preterm
- Early term birth (37-38 weeks) is NOT considered premature but does carry increased risks compared to full term (39-40 weeks) 7, 8
- The major clinical transition in terms of special care needs occurs between 34 and 37 weeks 6
Viability Threshold
- Human viability is currently approximately 23-24 weeks in developed countries, defined as the gestational age at which survival chance reaches 50% 4
- Special preparations are required for any anticipated preterm delivery, as these infants have immature lungs, increased bleeding risk, temperature instability, and infection susceptibility 1