Objective: To investigate associations between folate-metabolizing gene variations, folate status, and unexplained female infertility.

Design: An association study.

Setting: Hospital-based IVF unit and university-affiliated reproductive research laboratories.

Patient(s): Seventy-one female patients with unexplained infertility.

Intervention(s): Blood samples for polymorphism genotyping and homocysteine, vitamin B12, and folate measurements.

Main outcome measure(s): Allele and genotype frequencies of the following polymorphisms: 5,10-methylenetetrahydrofolate reductase (MTHFR) 677C/T, 1298A/C, and 1793G/A, folate receptor 1 (FOLR1) 1314G/A, 1816delC, 1841G/A, and 1928C/T, transcobalamin II (TCN2) 776C/G, cystathionase (CTH) 1208G/T and solute carrier family 19, member 1 (SLC19A1) 80G/A, and concentrations of plasma homocysteine, vitamin B12, and serum folate.

Result(s): MTHFR genotypes 677CT and 1793GA, as well as 1793 allele A were significantly more frequent among controls than in patients. The common MTHFR wild-type haplotype (677, 1298, 1793) CAG was less prevalent, whereas the rare haplotype CCA was more frequent in the general population than among infertility patients. The frequency of SLC19A1 80G/A genotypes differed significantly between controls and patients and the A allele was more common in the general population than in infertile women. Plasma homocysteine concentrations were influenced by CTH 1208G/T polymorphism among infertile women.

Conclusion(s): Polymorphisms in folate pathway genes could be one reason for fertility complications in some women with unexplained infertility.