I agree with Loula, tell them it's a hospital appointment, they have no right to ask what it for, but they can ask to see proof of the appointment, so you can always ask your doctor to confirm it in writing without saying what it was for....if that makes sense?!
Hope that helps!
I'm in a similar situation, just started a new job and found out I am pregnant. As long as you didn't know about it at interview stage then it's ok. If you know at interview stage, or when they offered you the job I think you are supposed to declare it.