A retrospective review of 128 consecutively scanned pregnant patients who had either early intrauterine pregnancies (IUPs) or ectopic pregnancies was performed to determine the accuracy of the finding of an intrauterine double decidual sac (DDS) in distinguishing between these two groups of patients. The presence of a DDS correlated with an IUP in 59 of 60 patients (98.3%). Of 68 patients who lacked a DDS, only four had normal IUPs; the remaining patients had either ectopic pregnancies or abnormal IUPs. Results of this study suggest that the finding of a DDS at ultrasonography is useful in making an early diagnosis of IUP, while its absence strongly suggests either an ectopic pregnancy or an abnormal IUP.