Not knowing your co-workers, it is of course difficult to predict how they will respond.
However, in my experience most employers and co-workers do not expect you to put your personal life on hold because of things going on at work. Ask yourself how you would feel if you found out one of your co-workers were to become pregnant.
Should the question come up, I would be honest with your co-workers and say yes, we were trying. You might give them a further explanation if you feel it is warranted, such as : We had decided a long time ago that we would start trying now, it was a little harder to begin trying knowing the problems it would cause at work, but at the same time we realized that there would always be some reason not to start trying.
If a co-worker decides to be angry about it, I don't think there will be much you can say or do to change their mind, but it is likely that someone like that will find something to be upset with anyways. Most people will be delighted for you I'm sure.
Also, keep in mind, it often takes months to get pregnant, so by the time you are pregnant it is quite possible that things might have changed at work.