I was having problems with another employee prior to my discoveries. She had hit me and I was in the middle of fighting that battle because the employer said we still had to work side by side and I was requesting we be separated since I had enough of this woman. I was actually trying to work on being moved out of the department, however believe my boss was working against that move. I think I was still there as part of her personal control issues and refusal to show her boss that she had lost an employee. She is now retired and I am sure to this day she probably tells everyone how everyone loved her.
Anyway, I was speaking to the head of HR when I discovered I was pregnant. I didn't know what to do at this point, but the woman I was having trouble with tried to play some sort of stupid game of chicken with me in the hall and I was tired of the harassment. So, I sent the head of HR an email stating I was pregnant and was afraid of losing my husband first baby in the stress of the mess and or dodging the woman so that I couldn't be accused of touching her. As well, I added all of the history in the email, much like a newspaper will do when telling a story even though everyone already knows the history of the past story. No sooner did I hit the send button, did my phone ring and the head of HR ask me to come see him.
Suddenly, I was told someone saw the woman do that do me and she was given two days off and it was filed in her report.
I requested (due to my age and personal request) that they not make any announcement, as I would do that when I was ready. It killed my boss. She kept telling she didn't know what the big deal was. It was now my turn to toy with her. It was my business and there are privacy laws in place.
So in my case, it worked to my advantage, but it is your business and you can tell them whenever you want or not at all.
Just take your time and think it out well in what you plan is. Once they realize you are working on protecting yourself and not sharing with them out of emotion, they back off because it is not worth it to them to set themselves up for a lawsuit.