According to current estimates, about 1 out of 1000 pregnancies is complicated by neoplastic disease. Incidence of cancer during pregnancy shows a growing trend, as women decide to procreate at a later age. Early and reliable diagnosis is crucial in cancer treatment. Several modalities are used to detect a neoplasm. In the setting of pregnancy, diagnostic possibilities are very limited. Pregnancy-associated physiological alterations occurring in female organism may influence some of the tests, therefore their reliable interpretation requires considerable experience on the part of the clinician. Furthermore, so diagnostic techniques can not be applied during initial gestational weeks due to their deleterious impact on the fetus. Also, some tumor markers may be useless. Fetus-safe diagnostic modalities include: cytological smear, colposcopy, sonography, manual palpation, colonoscopy, lab tests, fine needle biopsy, bone marrow biopsy and lumbar puncture. Most of them are routinely used, also in pregnant women. Some techniques are safe, if used cautiously. With adequate preventive measures, they may be considered harmless for both mother and fetus. These include: guided biopsy, conization of cervix, magnetic resonance imaging, mammography and, in selected cases, radiography. Diagnostic modalities contraindicated during pregnancy are mainly radiography, computed tomography, laparoscopy (in some authors’ opinion) and studies involving administration of radioactive isotopes.