The purpose of this study was to evaluate whether the MTHFR variants, folate and vitamin $B_{12}$ deficiencies increase the risk of hyperhomocysteinaemia and adverse pregnancy outcome such as short gestational age or reduced birth weight. Healthy pregnant women (n=136; 24-28 gestational weeks; 20-40 years old), who visited Ewha Womans University Hospital for prenatal care, participated in this study. At the time of delivery, trained nurses recorded the pregnancy outcome from medical chart. We determined maternal MTHFR polymorphisms (C to T subsitution at nucleotide 677) and measured serum homocyteine, vitamin $B_{12}$, and folate concentrations. We compared serum homocysteine level by MTHFR genotype, serum folate and serum vitamin B12 levels using ANOVA. To evaluate the association between serum homocysteine level and pregnancy outcome, we compared the gestational age and birth weight by serum homocysteine levels using multiple regression analysis, adjusting for other potential predictors. Mean level of serum homocysteine was highest among pregnant women of the MTHFR variants with low levels of serum folate and vitamin $B_{12}$. Regarding association with birth outcome, we found the relationship between homocysteine levels and increased gestational age (p=0.03) and reduced birth outcome (p>0.05). Our data demonstrates that serum level of folate and vitamin $B_{12}$ among pregnant women affects significantly serum homocysteine levels, and the genetic polymorphism of MTHFR modulates the relationship between them. However, we did not have conclusive evidence of association between high homocysteine level and adverse pregnancy outcome such as preterm or low birth weight.