Public health guidance recommends 400–800 mcg of folic acid or folate daily for anyone who could become pregnant, starting at least one month — ideally three — before conception. The neural tube closes around week four of pregnancy, often before a missed period.
Three months is also roughly the window in which developing eggs mature, which is why many fertility protocols use a 90-day runway for nutritional support generally.
A prenatal is a supplement to food, not a replacement for it. Look for adequate folate, iodine, vitamin D and iron, and check any additional products you take so you do not double up on the same nutrients — particularly vitamin A.