Heterotopic pregnancy is a condition characterized by implantation of one or more viable embryos into the uterine cavity while another one is implanted ectopically, particularly into the uterine tube. Its occurrence has increased drastically over the last few years due to assisted reproduction procedures. In general, the diagnosis is made during the first trimester by using endovaginal two-dimensional ultrasound (2DUS), through observing a complex para- or retrouterine mass in association with a viable uterine pregnancy. However, under some conditions such as atypical ultrasonographic presentations, 2DUS does not clarify the situation whereas magnetic resonance imaging (MRI) of the pelvis is able to do so. We present the case of a pregnant woman in her fifth pregnancy, with a clinical condition of lower abdominal pain and pallor. Endovaginal 2DUS showed a complex voluminous mass in the left pelvic region in association with a viable intrauterine pregnancy. 2DUS in power Doppler mode showed "ring" vascularization, compatible with an ectopic gestational sac. MRI was of great importance in that it suggested that the mass had hematic content, which together with the clinical features, indicated that laparotomy should be performed. This surgical choice was essential for the woman to achieve a clinical improvement and for good continuation of the intrauterine pregnancy.