Female cancer patients in the reproductive years face a variety of diagnostic and therapeutic procedures which pose teratogenic hazards to an unexpected pregnancy. Ionizing radiation, diagnostic biopsy and surgical resection procedures, anesthesia, chemotherapeutic agents and nuclear medicine scans are commonplace interventions which are contraindicated in the presence of a pregnancy, yet may be required for cancer diagnosis and treatment. Woman facing such procedures are tested for pregnancy so that decision-making regarding pregnancy and the need for cancer treatment can be made prospectively, and to avoid potential iatrogenic harm, emotional distress for the patient, and liability for the physician. There are substantial knowledge gaps amongst physicians regarding the limitations of the different tests. It is critically important that the physician understand the types of available human chorionic gonadotropin (HCG) tests; the differences in sample types (urine or serum); and the locations where the tests can be performed. Analytical sensitivities differ between urine and blood HCG tests, which yield differences of several days in pregnancy detection. Reasons for erroneous urine HCG results will be examined. Variability between measurements, test interferences and HCG variants will be discussed. The clinical impact of using the right test on the right sample at the right time on sexually active female cancer patients in their reproductive years will be highlighted.