Different pathways for preterm birth between singleton and twin pregnancies: a population‐based registry study of 481176 nulliparous women

ObjectiveTo explore the contribution of pregnancy‐related complications on the prevalence of extremely, very and late preterm births in singleton and twin pregnancies. To study the risk of spontaneous preterm birth in twin pregnancies compared with singleton pregnancies.DesignPopulation‐based registry study.SettingMedical birth registry of Norway and Statistics Norway.PopulationNulliparous women with singleton (n = 472 449) or twin (n = 8727) births during 1999–2018.MethodsPrevalence rates of pregnancy‐related complications for extremely, very and late preterm birth in twin and singleton pregnancies were calculated with 95% confidence intervals. Multivariable logistic regression was applied to assess odds ratios for preterm birth, adjusted for obstetric and socio‐economic factors.Main outcome measuresExtremely preterm (