Hi Spiderbee, I have often wondered this as I am 24, healthy and fit and have no family history of spina bifida, yet I was encouraged by my midwife to take folic acid up to week 12 of my pregnancy (I am now week 18). Obviously at the moment I do not know if baby has SB or not, so cannot answer from that aspect, but as I found out I was pregnant at 6 weeks I only took the tabs, when I remembered them, for 6 weeks and wondered what difference this would make.
We decided against having the SB / Downs test because midwife said I was in a low risk group. I don't believe in taking supplements unnecessarily so would be interested to hear other people's opinions, like Daffy's, for example; I was discouraged by my GP and midwife from taking iron supplements due to it having negative effect on mum and baby, yet 20 years ago you were told to take them!