Abstract： Objective:To analyze the clinical features, treatment, and outcomes of fetal/neonatal atrial flutter (AFL) at the onset of the perinatal period to improve the management of this condition.Methods:This retrospective study analyzed the clinical data, treatment, and follow-up results of fetal/neonatal AFL cases transferred to Shanghai Children's Medical Center from November 2013 to August 2021. Clinical characteristics, cardioversion procedures, and outcomes were summarized. Descriptive method was used for statistical analysis.Results:A total of 21 fetuses/neonates presenting with AFL in the perinatal period were involved in this study, including 17 males and four females. Ten of them were born at full term, and 11 were preterms. All of the patients were delivered by cesarean section at 32 to 41 gestational weeks [ (36.6±1.9) weeks] with a birth weight of 2 130 to 4 450g [ (3 059±528) g]. Increased fetal heart rate was all detected after 32 weeks of gestation, and three of them were diagnosed with AFL by fetal echocardiography before being born. The heart rate remained elevated in all cases after birth. All were diagnosed as AFL based on an electrocardiogram on the day of birth, which showed a 2 to 6 over one ratio of atrioventricular conduction. Among the six cases of cardiac insufficiency and low blood pressure complicated by dyspnea and cyanosis, the symptoms were relieved in four cases after mask oxygenation and two cases after ventilation. Among the 21 cases, one was converted spontaneously to normal sinus rhythm and the other 20 recovered after medication or electrical cardioversion. Seven cases were initially treated by drug conversion with a success rate of 5/7 and hospitalized for 23 d (13-25 d). There was one with cardiac insufficiency before treatment and three newly developed cardiac insufficiency during treatment among the seven cases. Thirteen cases were offered electrical cardioversion initially, and the success rate of cardioversion was 12/13. There were five cases of cardiac insufficiency before treatment, while no new cases of cardiac insufficiency was reported during treatment. The duration of hospitalization was 11 d (9-14 d). Apart from one case, the rest 20 infants were followed up from one month to eight years old, and no recurrence was reported.Conclusions:For fetal/neonatal AFL with the onset during the perinatal period, the symptoms mainly manifest in late pregnancy. Its diagnosis depends on fetal echocardiography before birth or electrocardiogram after birth, and electrical cardioversion is a fast and effective measure. While the prognosis of perinatal-onset AFL is generally good.