Pregnancy lasts for about 280 days or 40 weeks. A preterm or premature baby is delivered before 37 weeks of your pregnancy. 1  Extremely preterm infants are born 23 through 28 weeks.2  Moderately preterm infants are born between 29 and 33 weeks.3  Late preterm infants are born between 34 and 37 weeks.regnancy lasts for about 280 days or 40 weeks. A preterm or premature baby is delivered before 37 weeks of your pregnancy. 1  Extremely preterm infants are born 23 through 28 weeks.