The incidence of masses in pregnancy is estimated to occur in 1/81 to 1/2,500 pregnancies. The development of colorectal carcinoma during pregnancy is a more rare event, with less than 30 cases above the peritoneal reflection reported in the last 70 years. The differential diagnosis of mucinous adenocarcinoma of ovarian vs. gastrointestinal origin is often difficult. We report a pregnant patient affected by advanced colorectal cancer, who presented with an asymptomatic unilateral adnexal mass on ultrasound. A 28-year old woman was referred to our hospital after a routine ultrasound examination at 26 weeks gestation showing a right adnexal mass. At elective exploratory laparotomy, the patient was found to have metastatic mucinous adenocarcinoma. Diagnostic and treatment choices of such a cancer in a pregnant patient were explored. The final diagnosis of colorectal cancer was made only at the time of a subsequent emergency laparotomy. The goal of an obstetrician/gynecologist and other care givers of pregnant patients, is to achieve a healthy mother and child. Unfortunately, physicians may unwillingly sacrifice the health of the mother by denying or delaying her procedures or treatments simply because she is pregnant. It is especially important in the case of adnexal masses and their related pathology, due to the difficulty in detection and management of such cases during pregnancy, that doctors actively assume the responsibility of assuring that pregnant patients receive the proper care they need.