['This study was undertaken to determine whether a cytosine to thymine mutation at nucleotide 677 in the gene encoding for methylenetetrahydrofolate reductase is associated with particular subtypes of recurrent unexplained spontaneous abortion.', 'The prevalences of cytosine to thymine mutation at nucleotide 677 in the gene encoding for methylenetetrahydrofolate reductase among 41 patients with recurrent unexplained spontaneous abortions and among 18 healthy control subjects were determined with polymerase chain reaction.', 'Homozygosity and heterozygosity for the cytosine to thymine mutation at nucleotide 677 in the gene encoding for methylenetetrahydrofolate reductase were observed at nonsignificantly different rates among patients and control subjects (9% and 48% versus 22% and 38%, respectively, P <.95). Among patients with recurrent unexplained spontaneous abortions both homozygosity and heterozygosity were associated with significantly increased prevalence of recurrent early fetal loss rather than with repeated anembryonic gestations (P <.0001).'] Is polymorphism for mutation of cytosine to thymine at location 677 in the methylenetetrahydrofolate reductase gene associated with recurrent early fetal loss?