It depends how desperate you are for the job hun. I have tbh and say if it was me i probably wouldn't tell them until after i'd got the job.
I used to employ people and i must confess if i had two people attend interviews for the same job that were on a par with experience and education and i thought they would both be able to do the job, if one was pregnant and one wasn't then i would hire the non-pregnant one as it means you don't have so many things to deal with. ie. replacement during maternity leave, risk assesments etc.

I'm not saying i wouldn't have hired a pregnant woman at all. If she was best for the job then so be it, but you would be at a disadvantage. Also it is worth bearing in mind you wouldn't get maternity pay from that job as you have to have been working there before you conceived to qualify although you would qualify for government maternity pay. Xx