I told my (soon to be) new boss very early, but only because I had applied for an internal promotion with her (in a completely different area of my job), and when she offered it to me I felt like I should be up front about my pregnancy as it impacts some of my responsibilities and it felt weird accepting the job and then telling her a couple of months later. It went great! She smiled immediately and said "congratulations!" and assured me the new job would be flexible enough to accommodate appointments and that I'd still be entitled to all leave time with the transfer. I also expressed some concerns about starting a new job when fatigued and feeling sick and she laughed and said "I've been pregnant." I'm still pretty nervous about starting while newly pregnant, but I felt so reassured by that conversation. Later that night she emailed me an article about New Zealand's PM being pregnant. She seems super supportive.