Preterm infant is defined as -

Preterm is defined as babies born alive before 37 weeks of pregnancy are completed. Subcategories include- 1. Extremely preterm is less than 28 wks 2. Very preterm 28 to 32 wks 3. Moderate to late preterm 32 to 37 wks Reference Park&;s textbook of preventive and social medicine 23rd edition pg 536