Sari Räisänen

Objective To 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. Design Population‐based registry study. Setting Medical birth registry of Norway and Statistics Norway. Population Nulliparous women with singleton ( n = 472 449) or twin ( n = 8727) births during 1999–2018. Methods Prevalence 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 measures Extremely preterm (<28 +0 weeks of gestation), very preterm (28 +0 –33 +6 weeks of gestation) and late preterm (34 +0 –36 +6 weeks of geatation) birth. Results Preterm birth was significantly more prevalent in twin pregnancies than in singleton pregnancies in all categories: all preterm (54.7% vs 6.1%), extremely preterm (3.6% vs 0.4%), very preterm (18.2% vs 1.4%) and late preterm (33.0% vs 4.3%) births. Stillbirth, congenital malformation and pre‐eclampsia were more prevalent in twin pregnancies than in singleton pregnancies, but the prevalence of complications differed in the three categories of preterm birth. Pre‐eclampsia was more prevalent in singleton than in twin pregnancies ending in extremely and very preterm birth. The adjusted odds of spontaneous preterm live birth were between 19‐ and 54‐fold greater in twin pregnancies than in singleton pregnancies. Conclusions Singleton and twin pregnancies seem to have different pathways leading to extremely, very and late preterm birth.