I had my first appointment with the MW this morning and discussed this very thing, funnily enough! She said that you can take it past 12 weeks, it won't do any harm, but the positive benefits only really apply to foetal spine development in the first 12 weeks so taking it beyond this period doesn't really achieve anything. But it's not harmful!
Its not harmful and it is actually advised for certain groups of people to take it throughout pregnancy. There are benefits to taking later on but I'm not sure what they are (Dr gave me some reason when she told me to keep taking itit but I've forgotten now).
The first 12weeks are the really important time to take it though.
Not everyone has to take them can I just add for those saying 'it's very important' my doctor recommended that due to my anorexia I don't take them she said the chances are low of it affecting my baby but due to my weight I would have more chance of having a miscarriage if I took the folic acid supplements and recommended i just ate foods rich in folate