In women with heart disease, sustained arrhythmias can result in an increased risk to the mother and fetus. The purpose of this study was to determine the recurrence rates of arrhythmias during pregnancy in women with cardiac rhythm disorders and examine the impact on fetal and neonatal outcomes. Women with tachyarrhythmias before pregnancy who underwent obstetric care at the Toronto General and Mount Sinai Hospitals from 1990 to 2002 were included. The recurrence rates of arrhythmias were calculated. A multivariate logistic model was used to identify predictors of fetal complications. Seventy-three women had 87 pregnancies; 36 pregnancies were in women with a history of paroxysmal supraventricular tachycardia, 23 with paroxysmal atrial fibrillation or atrial flutter (AF/Afl), 6 with persistent AF/Afl, and 22 with ventricular tachycardia. In the women in sinus rhythm at baseline, 44% (36 of 81 pregnancies) developed recurrences of tachyarrhythmias during pregnancy or in the early postpartum period. The specific recurrence rates during pregnancy in women with a history of supraventricular tachycardia, paroxysmal AF/Afl, and ventricular tachycardia were 50%, 52%, and 27%, respectively. The 6 women in AF/Afl at baseline remained in this rhythm throughout their pregnancy. Adverse fetal events occurred in 17 of the 87 pregnancies (20%). Adverse fetal events occurred more commonly in women who developed antepartum arrhythmias (RR 3.4, 95% confidence interval 1.0 to 11.0, p = 0.045) compared with those who did not. In conclusion, in women with preexisting cardiac rhythm disorders, exacerbation of arrhythmia during pregnancy is common. Recurrence of arrhythmia during the antepartum period increases the risk of adverse fetal complications, independent of other maternal and fetal risk factors.