I work for the NHS as well and you can't be discriminated against for being sick and pregnant eg . sacked whilst on sick but if you've been on sick for a while, your pay will drop. If you are off sick forlong enough, eventually you will go on to early mat leave and then get your full pay for so many weeks, half pay etc.......
as everyone has said, you need to check your contract or speak to HR.
I guess it all depends on how long you've been off sick.
hth
Rachael
xxx