Catalyzes the reductive methylation of 2'-deoxyuridine 5'-monophosphate (dUMP) to thymidine 5'-monophosphate (dTMP), using the cosubstrate, 5,10- methylenetetrahydrofolate (CH2H4folate) as a 1-carbon donor and reductant and contributes to the de novo mitochondrial thymidylate biosynthesis pathway. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + dUMP = 7,8-dihydrofolate + dTMP Pyrimidine metabolism; dTTP biosynthesis. Homodimer. Expressed both in normal and cancerous tissues. Expressed only in cancerous tissues. Belongs to the thymidylate synthase family.