The preterm deliveries in the Norwegian Mother and Child Cohort (MoBa) study were divided into subgroups: spontaneous preterm delivery, iatrogenic preterm delivery, multiple pregnancies, congenital malformations and intrauterine fetal deaths (IUFD). Records were linked with the Medical Birth Registry of Norway in order to permit identification of the different subgroups. The first 53,711 included pregnancies in the MoBa study were analyzed. Spontaneous preterm delivery accounted for 42% and iatrogenic preterm delivery for 28% of all preterm deliveries. Other important contributors to preterm delivery were multiple pregnancies (16%), congenital malformations (8.6%) and IUFD (5.2%). In conclusion, the subgroup composition of the preterm population in the MoBa study is similar to previously published data regarding both different and similar populations and is well suited for future studies of this complex obstetric entity.