Objectives <p>Adenomyosis is a common gynecologic disease involving the uterus, with its effect on pregnancy being poorly understood. This study aimed to evaluate the associations between adenomyosis and obstetrical and newborn outcomes.</p> Methods <p>Using the Healthcare Cost and Utilization Project-National Inpatient Sample from the United States, we conducted a retrospective cohort study of all birth-related admissions from 2016 to 2019. Pregnancies with adenomyosis were identified using the ICD-10 code N80.03, with the remaining pregnancies being the reference group. Then, ICD-10 codes were used to identify obstetrical and neonatal outcomes and multivariable logistic regression models, adjusted for baseline maternal demographics, were used to determine the impact of adenomyosis on these outcomes. Also, in light of the potential detection bias inherent in births by cesarean delivery, adjustment for mode of delivery was included in all regression analyses.</p> Results <p>Among the 2,943,532 women who delivered between 2016 and 2019, 1,084 had adenomyosis, for an overall prevalence of 36 cases/100,000 births, which was stable throughout the study period. Adenomyosis in pregnancy was associated with increased frequency of placenta abruptio (adjusted OR 1.7, 95% CI 1.2-2.4), preterm delivery (1.4, 1.2-1.6), preterm premature rupture of membranes (1.3, 1.1-1.6), postpartum hemorrhage (2.7, 2.1-3.3), post-partum transfusion (2.2, 1.6-3.0), disseminated intravascular coagulation (9.3, 4.2-20.9), sepsis (2.7, 1.6-4.5), congenital anomalies (2.0, 1.3-2.8), and intrauterine fetal demise (2.0, 1.0-3.8). Also, these pregnancies had an elevated risk of delivering by cesarean (15.7, 12.7-19.3).</p> Conclusion <p>Adenomyosis in pregnancy is associated with adverse obstetric and fetal outcomes. As such, pregnancies in patients with adenomyosis should be considered high-risk and should be delivered in centers capable of managing the potential poor events associated with these pregnancies.</p>