I kept my first pregnancy very quiet for a long time, although it seems like half the workforce of my partner's employer learned about it very quickly indeed. I told dp and mum almost immediately I realised (5/6 weeks), union convenor at 9 weeks (before holiday and after a rather worrying announcement of further dramatic restructures at work), line manager with permission to tell certain other named individuals who needed to know. Then I told my boss's boss's secretary, and said I would give her permission to gossip at the right time (I told her she could let others know at about 5 months!) Because I barely showed until very late in pregnancy, I was able to pretty much choose who I wanted to tell and when.
But I was helped in this by very supportive partner and mother as well as by mumsnet. Whether or not you have other support, I have to echo everyone who recommends joining the antenatal thread for your due month, I found mumsnet v helpful early on and then dropped out, but was made welcome in my last weeks of waiting for baby to arrive, and don't know what I would have done without it during those horrible slow days, or now as I adjust to the shock of first time parenthood.
I was also on the April 2007 antenatal thread, though baby didn't arrive until May.
Whatever you worry about or experience, you'll find people on mumsnet who've been there and that really does make a difference.
Hope all goes well for you, take care, hope to see you around on the forums.