Pregnancy is a period of profound physiological transformation, marked by significant hormonal shifts and immune adaptations in both the mother and developing fetus. This dynamic interplay influences a critical window known as the first 1,000 days—from preconception through the first two years of life—a time recognized for its lasting and often irreversible impact on offspring health. The infant’s first year is especially pivotal for immune development and neurodevelopment, shaping susceptibility to infections and immune-mediated diseases. This cascade of developmental events is profoundly influenced by maternal health and environmental exposures, including nutritional status. Among the most impactful factors is maternal vitamin D status. Beyond its classical role in calcium homeostasis, vitamin D is a potent regulator of immune and genetic processes. Uniquely during pregnancy, there is a direct relationship between maternal circulating 25(OH)D and 1,25(OH)₂D concentrations, underscoring its physiological importance in this life stage. Randomized controlled trials have consistently shown that higher-dose vitamin D supplementation during pregnancy (4,000–4,400 IU/day) is associated with reduced risks or positive trends in the prevention of preeclampsia, preterm birth, gestational diabetes, and infections. To accurately assess these effects, it is essential to include studies in which all participants receive the ethical standard of prenatal care in high-income settings—typically 400 IU/day via prenatal vitamins. Exclusion of such studies from meta-analyses diminishes statistical power and introduces bias, weakening our understanding of vitamin D’s impact. Reevaluating landmark clinical trials with this in mind is critical, particularly in higher-income countries like the United States, where maternal health outcomes are comparatively poor. While vitamin D supplementation alone may not fully reverse maternal morbidity, compelling evidence suggests that achieving maternal 25(OH)D concentrations of at least 100 nmol/L can meaningfully reduce risk. When combined with other health-promoting factors—such as balanced nutrition, regular physical activity, and a safe environment—vitamin D sufficiency contributes to improved maternal and infant outcomes. Recognizing maternal vitamin D status as a sentinel marker of overall maternal health is a vital step toward optimizing pregnancy care and lifelong health for the next generation.