The prenatal period represents a critical window of susceptibility to inorganic arsenic (iAs) exposure from contaminated drinking water. Ingested iAs undergoes hepatic methylation generating mono and di-methyl arsenicals (MMAs and DMAs, respectively), a process that facilitates urinary arsenic (As) elimination. Differences in pregnant women's metabolism of As as indicated by greater proportions of MMAs and smaller proportions of DMAs in urine are a risk factor for adverse birth outcomes. One carbon metabolism (OCM), the nutritionally-regulated pathway essential for supplying methyl groups, plays a role in As metabolism and is understudied during the prenatal period. In this cross-sectional study from the Biomarkers of Exposure to ARsenic (BEAR) pregnancy cohort in Gómez Palacio, Mexico, we assessed the relationships among OCM indicators (e.g. maternal serum B12, folate, and homocysteine (Hcys)), and levels of iAs and its metabolites in maternal urine and in neonatal cord serum. The prevalence of folate sufficiency (folate levels > 9 nmol/L) in the cohort was high 99%, and hyperhomocysteinemia (Hcys levels > 10.4 μmol/L) was low (8%). However, 74% of the women displayed a deficiency in B12 (serum levels < 148 pmol/L). Association analyses identified that infants born to mothers in the lowest tertile of serum folate had significantly higher mean levels of %MMA in cord serum relative to folate replete women. In addition, elevated maternal Hcys was associated with total As in maternal urine and cord serum as well as cord serum %MMAs. The results from this study indicate that maternal OCM status may influence the distribution of As metabolites in cord serum.