Ohh just found the answer online:

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

Thanks girls, sorry Bexxie - didn't see your reply as I was posting at the same time.

K, will pm you