Pregnancy anemia is a global health problem often associated with low birth weight (LBW). The prevalence of anemia among pregnant women in Indonesia was 27.7% in 2023, while West Sumatra reported 11.26% in 2024. Low maternal hemoglobin (Hb) levels are thought to reduce the supply of oxygen and nutrients to the fetus, although the evidence remains inconsistent. This study aimed to determine the relationship between first-trimester maternal Hb levels and birth weight. This retrospective cohort study used secondary medical record data collected in 2019–2021 in Padang Panjang City, Agam Regency, and Tanah Datar Regency, analyzed in 2026. Using total sampling, 80 mother–infant pairs meeting the inclusion criteria were included. Maternal Hb level (anemia <11 g/dL) was the independent variable, birth weight was the dependent variable, controlled for maternal age, education, and occupation. Data were analyzed using the Shapiro–Wilk test, independent t-test, and multiple linear regression (α = 0.05). Of the 80 pregnant women, 37.5% had anemia. The mean birth weight was 3187.06 ± 429.43 grams, with 2.5% classified as LBW. The mean birth weight did not differ significantly between anemic mothers (3195.83 ± 417.24 grams) and non-anemic mothers (3181.80 ± 440.68 grams; p = 0.889). Multiple regression confirmed no significant relationship between Hb and birth weight after adjustment (B = 32.32; p = 0.750); the overall model was not significant (F = 0.907; p = 0.481; adjusted R² = −0.006). No significant relationship was found between maternal Hb and birth weight, as most mothers had mild anemia or no anemia and received adequate antenatal care.