INTRODUCTION:: The upper limit of normal for the QT interval is 450 msec. A prolonged QT is associated with an increase in sudden cardiac death. Female gender, pregnancy and methadone use are risk factors for QT prolongation. This study sought to assess the QT interval in pregnant women undergoing methadone-maintenance therapy (MMT) in the third trimester and the factors that are associated with QT prolongation. METHODS:: We performed a retrospective review of pregnant women on methadone at Boston University Medical Center between June 2012 and May 2014. Corrected QT (QTc) from a third trimester electrocardiogram (ECG) was calculated using Bazettʼs formula. The urine toxicity screen (utox) done on the day of the ECG was reviewed. Paired t-test was used to compare QTc of those with cocaine-positive utox to the QTc of those with a negative utox. The relationship between QTc and methadone dose was assessed using Pearson correlation. RESULTS:: Of the 32 eligible pregnant women on methadone, 21 (65%) had an ECG performed. Their mean QTc was 428.7±33.9 msec. Pregnant women with a cocaine-positive utox had a longer QTc compared to those with a negative utox (476.0±31.7 msec vs. 420.8±28.6 msec, P = .04). There was no correlation between the dose of methadone (range 55 mg to 145 mg) and the QTc interval (p > .05). CONCLUSION:: The QT interval in pregnant women on methadone was below the upper limit of normal, except for those with a cocaine-positive utox. The concomitant used cocaine place pregnant women on methadone at risk of cardiac arrhythmia.