No longer a Matron of Honor

Sharon • Married Aug 2015. I'm 26 and expecting #1👶 July 2016!!
One of my closest friends is getting married in April and I just found out that I'm just over 5 weeks pregnant. I'm not sure when to tell her. A while back we spoke about the fact that if I was to get pregnant she would chose someone else as a replacement. I don't want to lie to her, but I know its too early to start telling people. Which of course she will have to , to find a replacement. I don't want to drag it out because that will be unfair to her! (Any brides nightmare ) Any advice ?