To tell or not to tell

I am four weeks pregnant with baby #1. Having never done this before, I'm finding the need to share my news with someone close to me who is also a mother and been through this journey. I still want to surprise our family and friends but do have one very close friend I'm thinking about telling the good news to who has been my sounding board all through TTC. What is your opinion on telling people you are pregnant and when to tell?