Pregnancy is a complex process that can be jeopardized when associated with cancer, because of the coexistence of two complex metabolic conditions: a fetus and cancer. The aim of this study was to evaluate fetal growth in association with cancer development as well as the indirect effects produced by tumors in pregnant mice subjected to a leucine-rich diet, knowing that leucine supplementation can minimize the tumor effects by acting as a cell signaling agent to improve the protein synthesis process. We evaluated fetuses (n = 6) from NMRI pregnant mice fed either a control or a leucine-rich diet in either the presence or absence of an MAC16 colon adenocarcinoma or ascitic fluid inoculation. The fetal serum amino acids were separated using high-performance liquid chromatography, and fetal cytokine levels were analyzed using a microsphere-based multiplex immunoassay (Luminex xMAP). Fetal body composition was measured as the water, fat, and protein total content and total serum protein, albumin, and glucose content. Tumor growth resulted in a severe reduction in fetal body weight and protein content and increased fetal resorption, associated with placental weight decrease; these effects were minimized by a leucine-rich diet. Serum total protein and glucose content were reduced in fetuses from tumor-bearing dams but were reverted by nutritional supplementation. The serum amino acid profiles differed significantly between the tumor-bearing mice fed with a leucine-rich diet and controls. Certain tumor effects were reproduced in fetuses from ascitic fluid-injected dams, suggesting indirect effects of tumor growth. We conclude that certain effects of tumor growth can be mimicked by ascitic fluid injection and can be modulated by a leucine-rich diet.