Why is folate supplementation recommended for women one month before conception and during the first trimester of pregnancy?

Folate can prevent birth defects of the baby's brain because it helps to form the neural tube.

Explanation:

Folate is the naturally occurring form of vitamin B9. After its intake, the digestive system converts it into the active form of vitamin B9 that will eventually be used by the cells to perform different functions. Basically, the body uses it to make new cells, because it is vital for cell growth and development. It is an important part the diet and it is also important during pregnancy. This is because it helps prevent neural tube defects such as anencephaly or spina bifida among others that occur when the spine and brain of a baby do not develop completely.  

So, as stated above, folate is used during pregnancy because it play a role in the development and proliferation of the the neural tube.