What is a premature baby?

http://en.wikipedia.org/wiki/Premature_birth

In humans, preterm birth (PB) refers to the birth of a baby of less than 37 weeks gestational age. Premature birth, commonly used as a synonym for preterm birth, refers to the birth of a premature infant. Because it is by far the most common cause of prematurity, preterm birth is the major cause of neonatal mortality in developed countries. Premature infants are at greater risk for short and long term complications, including disabilities and impediments in growth and mental development. Significant progress has been made in the care of premature infants, but not in reducing the prevalence of preterm birth.[1] The cause for PB is in many situations elusive and unknown; many factors appear to be associated with the development of PB, making the reduction of PB a challenging proposition.

Epidemiology
As weight is easier to determine than gestational age, the World Health Organization tracks rates of low birth weight (<>