Borderline ovarian tumors (BOTs) are common in women in their reproductive years. In more than one-third of patients tumors are detected at the age of 15–29, the average age at initial diagnosis is 40. The study was aimed to improve methods for BOTs diagnosis in pregnancy and to determine the possibilities of organ preservation treatment. A group of 300 pregnant women with various tumor-like formations and ovarian tumors was examined. Of them, 25 patients had borderline epithelial tumors (22 patients had serous and 3 patients had mucinous tumors). Ultrasound examination together with blood serum СА-125, sFas, VEGF and IL6 level assessment were performed prior to surgery. The results obtained were compared with the results of morphological studies. Organ preservation and radical surgical treatment were carried out, and chemotherapy, if necessary. Perinatal outcomes were studied when performing the cross-comparison. It was discovered, that ultrasonography and logistic regression analysis made it possible to distinguish between benign ovarian tumors, BOTs and malignant ovarian tumors. The levels of VEGF above the 500 pg/ml, IL6 above the 8.1 pg/ml and СА-125 above the 300 U/ml indicated the high probability of malignant ovarian tumors in pregnant women. Only the morphological study of ovarian tissue, obtained regardless of surgical methods, ensured understanding of the ovarian tumor’s true nature during pregnancy. At the same time, in three pregnant women with ovarian tumors, the morphological examination revealed some tissue areas common both for BOTs and malignant ovarian tumors. Thus, the predominance of the tumor early stages, relatively mild course and, favorable prognosis in patients with BOTs make it possible to use gentle surgical treatment making it possible to preserve menstrual function and fertility.