I'm of the opinion that you should be as open and honest with your employer, reason being is that the law is on your side anyway, they have to put your health first and will need to take steops to ensure you can do your job, whilst accomodating your needs i.e. appointments, sickness etc!
At least if you do let your work slip due to being ill, they will know there is a perfectly good reason for it!
When I was pregnant, I told my immediate boss, who I answer to, we kept it to ourselves untill I was 20 weeks, but at leasts he knew why I was a bit off-ish!