Morning Pierced I work quite closely with HR in my job and have a wee bit of experience here;
A risk assessment should be completed and then reviewed at regular intervals throughout your pregnancy. Otherwise they do put themselves and you at risk. Should anything happen to baby when they have not assessed your work and environment you would be more than entitled to sue the ass off them. True story.
As for illness, they cannot accumulate pregnancy illness alongside normal illness. So for instance, at my work they use the Bradford Factor mechanism to calculate a score based on the number of days, versus the number of instances of illness, when you reach 100 it triggers the HR meerkats; however if I was off with something pregnancy related, they are not allowed to add that into my Bradford factor.
ACAS is a fantastic website to consult as well as the others above. This is a quote from the section that you're asking about:
If an employee is off-sick because of her pregnancy, then she is entitled to sick pay in the normal way. (Antenatal care does not count as pregnancy-related illness in this respect.) Her pregnancy sickness should be recorded separately from other illness and should not be counted towards a total sickness record. Neither can pregnancy-related sickness absence be used as a reason for disciplinary action or redundancy selection.
Hope this helps.