Cancer is the second leading cause of death during the reproductive years, but the incidence during pregnancy is low. The incidence of cancer of the rectum during pregnancy is very rare (1:50,000 to 1:100,000 pregnancies). Usually, the symptoms of this type of cancer are nonspecific and sometimes they are confused with normal changes of pregnancy. The diagnosis of rectal cancer is usually made in the late stage of pregnancy. The treatment of this cancer involves surgery, chemotherapy and radiotherapy, but each type of treatment involves risks to the fetus and the pregnant woman. We present a case of a young primiparous woman who had a quick and bad evolution of an adenocarcinoma of the rectum diagnosed during pregnancy. This patient had extensive involvement of the liver with metastasis and she died two weeks after diagnosis. In this case, we present the clinical, radiological and pathology findings of this disease.