Can my employers complain if i have to take time off due to pregnancy?

Hi People, My employers have asked me to come back to work even though i am on a sick note until this coming weekend. I am suffering really badly from morning sickness and pain and have even had to stay in the hospital overnight on a drip due to malnutrition and dehydration!! i want to go back to my doctors and ask for another sick note as i am in no fit state to work. (at this point i should probably include that i am a cleaner and constantly have to bend twist and reach, hence why i cannot go back to work just yet as the slightest pressure on my stomach and i am throwing up) what i want to ask is can my employers demand i come back to work or stop paying me if i have more time off sick?? bearing in mind my employers aren't perfect when i told them i was pregnant 2 and a half months ago! they said they would do a risk esment but have not yet.. altogether so far i have had 4 weeks off pregnant due to start my 5th and need to take more time after. can they do this?? let me know asap please! thanks xx