I work for a fairly large company and had a former co-worker find out through a "confidential" conversation I had with a mutual "friend." She decided to tell a large amount of my immediate colleagues. I was pretty annoyed with all involved and myself.
Once I was over being outted by someone that I don't care for, it was easier for people just to find out when they find out.
Honestly most co-workers usually aren't my real friends and most won't ever see my child. I try to keep my personal life out of my business life however its hard with the belly. Its more about how you react, do you gush and tell details or do you confirm, thank, and move on?