We examined a young primipara with increased alpha-fetoprotein (AFP) values and cystic tumefaction of the right ovary. Having in mind a mild decrease in ovarian artery resistance index (RI) and suspected findings of fetal kidney, this situation was delicate due to its double pathology which was later confirmed. Wilms' tumor is the most common urogenital tumor in childhood, and it is detectable in the prenatal period by ultrasound examination. In utero kidney biopsy confirms diagnosis and facilitates decisions concerning the course of pregnancy. Relative risk of intervention limits this diagnostic procedure for indicated cases.