The day after I found out - I had taken the day off sick for a "tummy bug" which turned out to be bubba.

I work in a reasonably high risk situation, so made sense for close colleagues to know. I told the boss first, and gave her a moment to digest it. Then told other close colleagues, we were a permanent team of about 6. Told the rest of the workplace at 19 weeks, when I also submitted my letter of resignation (I don't want to go back to my permanent position, as it won't suit, so was best for both parties that I make that choice.)