Before pregnancy! If you are planning on getting pregnant then it is wise to start prenatal vitamins before you are pregnant to assure you are getting adequate Folic Acid to prevent a birth defect called neural tube defect. Most people don't know they are pregnant until the fetus is 2-3 weeks old. It is crucial to have adequate Folic Acid in those initial days of development, so don't wait until you know for sure!
Immediately. You should start prenatal vitamins when you start being prenatal.
Ideally, Before. Ideally, before pregnancy. Folic acid is one of the important (and often missing) vitamins needed during pregnancy. It is used in the development of the nervous system and brain, for which the first month of pregnancy is critical. Additionally, some women low in certain vitamins may have trouble getting pregnant, so the prenatal vitamins are beneficial while trying to get pregnant.
Folic acid. If you are considering getting pregnant, especially of you're actively trying, you should be taking a vitamin with at least 400mcg of folic acid. Many pregnancies are not detected until after the first month; this is a critical time for development of the brain and spinal cord. Folic acid helps to reduce birth defects like spina bifida.