Increased folate intake reduces the risk of neural tube defects, other malformations and also possibly, pregnancy complications. Increasing evidence suggests that the beneficial effect of folate may be related to improved function of methionine synthase, a vitamin B12-dependent enzyme that converts homocysteine to methionine. In India, the majority of the population adheres to a vegetarian diet known to be deficient in vitamin B12. In such a population, increased folate intake may offer minimal protection against birth defects, whereas vitamin B12 administration should be considered. In this review, is described the metabolism of and interrelations between folate, vitamin B12 and homocysteine. This is followed by a brief discussion of some of the proposed mechanisms for their biological effects in relation to birth defects and pregnancy outcome.