Folic acid (multivitamins) reduce the recurrence and occurrence of neural tube defects (NTDs). Vitamin profiles seem not suitable to identify women at risk for NTDs. A subset of these women have hyperhomocysteinaemia and a mutation of the gene for thermolabile methylenetetrahydrofolate reductase (MTHFR). From studies with the rat embryo in vitro, it can be concluded that the de- and remethylation cycle of methionine, being folate and vitamin B12 dependent, is crucial for embryonic and fetal growth probably via generation of DNA, proteins and polyamines. Nutrition for the embryo is also supplied by the follicular fluid, the yolk sac, the extraembryonic coelomic cavity and the amniotic fluid.