IMHO I really don't think it's a good idea to start dating someone new while pregnant. But if you really must, then I would say 2nd date is the time to tell him. Hmm you see now I am 50/50 whether you should tell him on first date....but anyway, definitely would tell him as soon as is appropriate.

Really though, I don't think dating anyone other than the father is a good idea...not to mention protecting your baby from STD's!