leaving work permanently for pregnancy

Okay so I'm very nervous about telling my employer that I'm pregnant in a couple weeks once first trimester is over. I work in a small office so it's literally just 3 staff members. We're already going through a big change in office so I feel like they're going to be upset more than anything to find out I'm pregnant. I plan on resigning and not coming back and that means that they need a definite replacement. It will be a lot of work to train and find someone new that the boss will feel comfortable with. I plan on staying til late march to help train and give them time to find someone. I just know it's going to be tough for them. But I need my relaxation time to really enjoy my third trimester alone with hubby before baby joins. Any tips or how I should come about it?