Hi DungunGirl, big concern for me too. I'm not planning to tell work until after my scan, or perhaps even later. We've just had a big re-organisation in our dept, which gave me quite a bit more seniority as well as a nice pay rise. Before I tell them I want 2 things - firstly to have all the paperwork sorted, so I know I'll be on the higher salary for maternity pay, and secondly, to have shown that I can handle all this new responsibility, and will be a valuable person to have back.
I know this sounds like I work in an awful place - that's not true at all. I have a great relationship with my boss, and she knew I was planning to start trying for bump 2 in the new year (oops, stupid condoms!!!). But like you, other mothers have slightly queered the pitch for me. One in particular had 2 very close together, basically did that thing of waiting exactly long enough to qualify again before getting pregnant again.
I might sound a bit anti-feminist/maternity/etc, and in a way I am. I have a relative who owns a small business who has been royally shafted by women in the past who knew how precarious his finances were, and how difficult it would be for him to support maternity leave, and yet joined the company anyway, thereby risking the livelihoods of others. So I've always been upfront with my boss about my plan to have another, and she's repaid my honesty with mutual respect.
Anyway, back on topic, that mother who had 2 close together also seemed to have the worst luck with her children's health (nothing major, just every single bug going) and made some very questionable choices on childcare (when your minder is diagnosed with cancer, of course support her emotionally but it might be worth finding a new carer when she's going through chemo, otherwise you end up letting down work a lot). She's now keeping her cards really close to her chest about what she's planning to do next - come back after 9 months, 12 months, or not at all, which has made it very hard for those here in a very small, close-knit dept to figure out how things will work out in future.
So, now I've bored you all to tears with my grumble, my advice is... tell work as late as you can, visible signs allowing, once you've sorted in your own mind what you want to happen. I think if you have a good relationship with your boss, it really helps if you're honest with her about how you see this playing out. If you don't think you'll be coming back, admit that. You don't have to tell senior management or HR, but your close colleagues will appreciate having a clearer view of the future and how it affects them.