I have been signed off with hyperemesis for the past 7 weeks. My employer has been fine about it.
You should know that any absences during pregnancy cannot by law count towards your sickness absence record - it's sex discrimination if it does.
Here is a useful factsheet - if your employer gives them any problems, print it off and give it to them ... useful quote:
'If you are ill during pregnancy normal sickness rules apply, with a few exceptions. If the illness is pregnancy related it must be recorded as such, it does not count towards your sickness record & you cannot be dismissed for it.'
Hope you feel beter soon.