<p>Maternal effects —non-genetic influences of the mother on her offspring phenotype— play a crucial role in shaping early development and fitness. In large mammals, these effects have been mostly studied in relation to maternal condition (e.g., age, body mass, dominance status) and environmental constrains, whereas the influence of social context has received far less attention. Moreover, fetuses are rarely used as direct indicators of maternal effects, despite providing valuable insights into prenatal investment before postnatal processes emerge. Using a cross-sectional dataset of over 1100 red deer (<i>Cervus elaphus</i>) mother-fetus pairs from multiple populations and years, we investigated how maternal, environmental, and social factors interact to shape fetal condition. We predicted that maternal age, condition, and early conception would enhance fetal development, whereas harsh weather would constrain it. Variations in population structure affecting the level of male-male competition allowed us to assess the influence of social environment. We found that fetal weight increased with maternal age and early conception date, particularly in males, highlighting advantages linked to maternal experience and condition. However, in populations with intense male-male competition, mothers produced lighter fetuses, especially adult and older mothers under unfavorable weather and in late gestation, suggesting a cost of social interactions with males on prenatal growth. Our findings reveal that maternal traits, ambient environment, and social factors jointly modulate maternal effects on fetal development, underscoring the value of fetal traits as indicators of maternal effects in mammals, and suggesting an unexpected potential negative effect of male intrasexual competition on maternal allocation in polygynous species.</p>