I just had to post because I have BEEN one of those other people before. lol
Most of your responses seem to be of the variety that tells you how to ignore what you are perceiving... but I think it isn't a matter of that... but a matter of the people around you maybe not knowing how YOU feel about it all... or even knowing if you ARE pregnant and not knowing a polite way to ask and feeling weird about not acknowledging something that may or may not be very obvious. People get all weird on you when they don't know what to do/say/acknowledge. If you address that, then you might find that nobody is weird around you anymore.
My pastor's wife just had baby number 5 a few months ago. I thought she was pregnant WAY before she was showing or told anyone... and I asked her... "are you pregnant?" (she was nauseous several days I had seen her within about a 3 week period) and she said "not that I know of"... so I didn't say anything else for WEEKS..... well, like 2 months. When it started to become obvious to me, but there wasn't an announcement made, I started wondering if there was a problem with the pregnancy, or if she was unhappy to be pregnant again or something. So I dreaded being rude by either not acknowledging something literally right there in front of me, or by asking about her being pregnant if she wasn't and I had been wrong about her increasing belly size, etc... Same token, another woman I am acquaintances with, looked to be a little larger and I thought... she looks pregnant... but she also had just had a baby, her 3rd, (less than 7 months before) and with each pregnancy had gained a little weight... so I was afraid to ask her b/c I honestly didn't KNOW and it would be way rude to ask if she was just getting big in the belly and NOT pregnant. I only saw her very infrequently and in groups where there wasn't a lot of personal chit-chat, and until the birth announcement arrived, I never was comfortable around her, b/c I just did not know if she was or wasn't.
I would make sure that a few close friends at work KNOW that you ARE and that you ARE very EXCITED about it. Tell them to feel free to share the wonderful news! I'll be there are a lot of your co-workers who will be relieved to know one way or the other. :)
(There might also be a few who are not thrilled, thinking about your post-delivery absence from work, and how that might impact them... but I wouldn't pay any attention to them about that. That is between you and your employer, not them.)