it can happen at any time as far as i know. i got it quite early on and seem to have gotten over it now - just short of 10 weeks (knock on wood). but my younger sister who is also preg at the moment, started to get sick around 12 weeks and it continued throughout her second trimester. my mum was telling me that for each of her six kids it was different. one she suffered no m/s and the others all at varying degrees and times throught her pregnancies.

so i guess it is just luck of the draw.