I am having difficulty finding the mechanism by which folic acid reduces the risk of neural tube defects. I know that it does so, but what in partic

I am having difficulty finding the mechanism by which folic acid reduces the risk of neural tube defects. I know that it does so, but what in particular actually occurs with folic acid deficiency to cause the neural tube defect? Is that completely unknown and merely a strong association, or is there a putative mechanism?