Hi Lucy,
Just seen this thread sowee
You can be signed off right up until 4 weeks prior to your due date, as when u hit the 4 weeks b4 ur due date you have to start maternity leave (providing the reason you are signed off is pregnancy related)
I was signed off from 6 months-have had a battle with HR this week who have refused me to go from sick to annual to mat.
PM me if u need any info xx