This is something I've debated too! I think i'm lucky because of teaching (like yourself!) I have finished work now until Sept 2010. Obviously I am on hol now but am hoping to either be signed off to rest by the doc or if he won't do it I'll start maternity leave in Sep. Looking back, I was so poorly last time. I was seriously swollen, dragging myself into work and feeling awful. I just do not need the extra pressure of a very hectic job when I need to concentrate on my baby! If I were you, 20 wks sounds like a good idea, I am really happy knowing that I don't need to get up in the morning!!
Lizzie