B.H.
N.,
First things first...congratulations! I had the same thing happen to me...well, not the dog part, but the rest of it. I did wait until I was about 3 months along. I was honest with them...it wasn't planned but it is a wonderful surprise, I wasn't sure whether or not I'd be coming back after the baby, but that I did want to keep working until I gave birth (btw-they can't fire you for being pregnant, it's illegal) and I would give them ample notice if I decided to quit so that they would have time to replace me and I could help train my replacment.
When it became clear that I was going to quit and become a stay at home mom, I told my bosses about it and we found a good replacment that I trained myself and left feeling good that I hadn't left them in a bad situation .
Actually about a month ago they called me and asked if I wanted to come back on a part time basis to help out with billing. It made it clear to me that I had done the right thing. I made the right decision for myself and my family, but I didn't burn the bridges with a fair and good employer.
By the way, I did go back to help them out two mornings a week and it has been great. The best of both worlds for me.
Good Luckm
B.