the first 12 weeks is the most important time. You can continue to take all the way through, but the increase in folic acid is needed during the first few weeks to prevent neural tube defects. The spinal column and all the nervous system bits (i believe) are fully developed by 12 weeks, so that's why it's not necessary to take it after (the idea being that it's too late by then anyway to do any good).
I read somewhere recently, or may have been on the news, that you should actually stop taking it b ythe time you get to your third trimester because the increase has been linked to osteoporosis...I can't remember but it's fairly recent that i heard it.
Best thing to do is check with your midwife or GP next time you see them.
xx