Introduction an absolute requirement for folate that is met entirely About 14% of individuals in Europe seek medical advice from endogenous sources. " Furthermore, the effective for infertility, of whom about half undergo in-vitro use of B vitamins is dependent on sequence variations fertilisation(IVF).1. Despite advances in the technology in the enzymes involved in their metabolism. Thus, used in IVF only a fifth of treatment cycles result in availability of B vitamins during the period immediately livebirths. Overall success of treatment seems largely preceding oocyte recovery and the B-vitamin related independent of the diagnosis of infertility or whether genotype of the mother or embryo might affect embryo IVF or intracytoplasmic sperm injection(ICSI)is used viability after IVF. The great majority of twins born after for fertilisation, 4s which suggests that factors other IVF are dizy gotic, resulting from the survival of more than those specific to the diagnosis-eg, poor sperm than one transferred embryo. Exposure to B vitamins quality-affect the success of treatment. To overcome might, therefore, also affect the incidence of twin births these unknown barriers and improve pregnancy rates, Our aim was to investigate the link between B-vitamin most IVF units transfer more than one embryo per intake and levels in the body and six variants in genes cycle. This practice increases the likelihood of multiple involved in B-vitamin metabolism, and the outcome of births and hence the risk of maternal and infant IVF mortality and morbidity. .8 The general factors that affect the success of ivf and the risk of twin births need Methods further research Participants Poor B-vitamin status and raised concentrations of Between October, 2000, and September, 2004, we did a homocysteine are associated with early loss of prospective cohort study to which we consecutively pregnancy, .1o whereas high concentrations of folic acid enrolled women undergoing fertility treatment at the have been associated with an increase in natural Aberdeen Reproduction Unit. We also obtained Dna twinning, though the latter connection remains from a separate cohort of women who were attending a controversial. Fertility probably also has a genetic pregnancy ultrasound scan at Aberdeen Maternity component, 1 4 and sequence variations in the genes Hospital. These women had conceived naturally and involved in the metabolism of B vitamins have been were carrying singleton pregnancies with no known linked to early pregnancy loss. 15. Little work has been abnormalities done on treatments for infertility per se, although there All women provided written informed consent, and are good biological reasons to suspect that B-vitamin Grampian Research Ethics Committee approved the exposure affects their outcome. The early embryo has study