Is she usually lazy? How long has she been working for you? Were you happy with her before?
Because, she didn't refuse to tidy up as soon as she told you she was pregnant.
What kind of tidying up takes her all day while the kids are at school? She can't be doing just that all day.
Anyway, it is not to any of us to say if she should tidy up or not. It is to her. Seriously, a pregnant woman tells you she finds it hard to clean for you, did you at leats ask her what was wrong and tried to find a solution? Or were you to busy being upset at her, because, obviously, the only reason she told you that was tha she is lazy and trying to take advantage of you.
Let's be overly dramatic, but what if she tidy up and clean for you, pass out and hurt herself/the baby. Are your eady to deal with the consequence of forcing your nanny to continue doing something after she told you that it was a problem for her
I think as an emplyer, if you are told about something like that, you need to make sure sure the employee is safe.