Background: Twin pregnancies carry a higher risk of congenital and structural malformations, and pregnancy complications including miscarriage, stillbirth, and intrauterine fetal death, compared with singleton pregnancies. Carrying a fetus with severe malformations or abnormal karyotype places the remaining healthy fetus at an even higher risk of adverse outcome and pregnancy complications. Maternal medical conditions or complicated obstetrical history could, in combination with twin pregnancy, cause increased risks for both the woman and the fetuses. To our knowledge, no previous studies have evaluated and compared the outcomes of all dichorionic twin pregnancies and compared the results of reduced twins with those of nonreduced and primary singletons in a national cohort. These data are important for clinicians when counseling couples about fetal reduction and its implications. Objective: This study aimed to describe and compare the risks of adverse pregnancy outcomes, including the risk of pregnancy loss, in a national cohort of all dichorionic twins—reduced, nonreduced, and primary singletons. In addition, we examined the implications of gestational age at fetal reduction on gestational age at delivery. Study Design: This was a retrospective cohort study of all Danish dichorionic twin pregnancies, including pregnancies undergoing fetal reduction and a large proportion of randomly selected primary singleton pregnancies with due dates between January 2008 and December 2018. The primary outcome measures were adverse pregnancy outcomes (defined as miscarriage before 24 weeks, stillbirth from 24 weeks, or single intrauterine fetal death in nonreduced twin pregnancies), preterm delivery, and obstetrical pregnancy complications. Outcomes after fetal reduction were compared with those of nonreduced dichorionic twins and primary singletons. Results: In total, 9735 dichorionic twin pregnancies were included, of which 172 (1.8%) were reduced. In addition, 16,465 primary singletons were included. Fetal reductions were performed between 11 and 23 weeks by transabdominal needle-guided injection of potassium chloride, and outcome data were complete for all cases. Adverse pregnancy outcome was observed in 4.1% (95% confidence interval, 1.7%–8.2%) of reduced twin pregnancies, and 2.4% (95% confidence interval, 0.7%–6.1%) were delivered before 28 weeks, and 4.2% (95% confidence interval, 1.7%–8.5%) before 32 weeks. However, when fetal reduction was performed before 14 weeks, adverse pregnancy outcomes occurred in only 1.4% (95% confidence interval, 0.0%–7.4%), and delivery before 28 and 32 weeks diminished to 0% (95% confidence interval, 0.0%–5.0%) and 2.8% (95% confidence interval, 0.3%–9.7%), respectively. In contrast, 3.0% (95% confidence interval, 2.7%–3.4%) of nonreduced dichorionic twins had an adverse pregnancy outcome, and 1.9% (95% confidence interval, 1.7%–2.1%) were delivered before 28 weeks, and 7.3% (95% confidence interval, 6.9%–7.7%) before 32 weeks. Adverse pregnancy outcomes occurred in 0.9% (95% confidence interval, 0.7%–1.0%) of primary singletons, and 0.2% (95% confidence interval, 0.1%–0.3%) were delivered before 28 weeks, and 0.7% (95% confidence interval, 0.6%–0.9%) before 32 weeks. For reduced twins, after taking account of maternal factors and medical history, it was demonstrated that the later the fetal reduction was performed, the earlier the delivery occurred (P<.01). The overall risk of pregnancy complications was significantly lower among reduced twin pregnancies than among nonreduced dichorionic twin pregnancies (P=.02). Conclusion: In a national 11-year cohort including all dichorionic twin pregnancies, transabdominal fetal reduction by needle guide for fetal or maternal indication was shown to be safe, with good outcomes for the remaining co-twin. Results were best when the procedure was performed before 14 weeks.