Heterotopic pregnancy is a life-threatening condition. A recent case at our institution prompted a systematic review of the literature from 2005 to 2010. In the majority (71%) of cases reviewed, risk factors for a heterotopic pregnancy were present. However, in several instances (33%), previous sonographic reports of a normal intrauterine pregnancy gave false reassurance. These results highlight the complexity of diagnosis. In addition, our findings were compared with two previous reviews covering cases from 1971 to 2004. This comparison highlighted two important trends: first, the increasing role of ultrasound in the definitive diagnosis of a heterotopic pregnancy, and second, the development of conservative approaches to management. Medical knowledge and technology may be improving, but ultimately, even in the presence of a known intrauterine pregnancy, the simple dictum 'think ectopic' must not be forgotten.