Methylenetetrahydrofolate reductase is an important enzyme in human folate metabolism, which converts 5,10-methylenetetrahydrofolate into biologically functional L-5-methyltetrahydrofolate and finally keeps homocysteine in human blood at a relatively low level through the methyl transfer pathway. Defects in the methylenetetrahydrofolate reductase gene can lead to disruptions in DNA replication, DNA methylation modifications, and cell cycle regulation, leading to cancer, cardiovascular disease, and neural tube development defects. Defective methylenetetrahydrofolate reductase in pregnant women may cause spontaneous abortion, hypertensive disorders during pregnancy, cleft lip and palate, congenital heart disease, and neural tube defects in the fetus. This test can be done if conditions allow for optimal birth and fertility.