A preterm birth is a birth occurring before the normal end or term of pregnancy. The normal duration of pregnancy is 40 to 41 weeks. Nonetheless, there are degrees of preterm birth; these are defined according to recommendations of the World Health Organization (WHO).
Gestational age is the criterion used to define preterm birth. Any child born before 37 completed weeks of gestation, or 8 months of pregnancy, is considered to be preterm or premature. Very preterm births are those that take place from the 5th through the 7th month of pregnancy.