I've been exactly where you are. I remember when my first dd was born, I wanted more than anything to have another within 18 months. Due to a few medical reasons I couldn't, but after about a year, I really talked myself out of the second. We waited until my dd was almost 2 before trying. So she is now 2yrs 8 months older than her sister. But we both worked full time and I knew we wouldn't be able to afford daycare for both. I was always told to quit waiting for the right time, because there never is a right time, there is always something that comes up. So getting pregnant because you want another baby is the only reason.
When I chose to stay home, our income was litterally cut in half. It was a huge change. But honestly we made it work. To tell the truth if my mom hadn't passed away at the same time and left me a little life insurance, we wouldn't have made it. But it was enough to pay off the car, so we didn't have that payment anymore.
We cut back cable, phones, extras, we stay home a lot, we enjoy free stuff (beach, boardwalk, parks, etc).
My best advice would be to start trying to cut back your spending now. Try living on a reduced budget and putting the extra money into a savings account. Work all the way through your pregnancy if possible and just keep adding to that savings. So when you do stay home without income, you will have a cushion to get you over the hump. It took about 4 months maybe 6, but we finally got it worked out and I've been home every since. Going on 4 years now.
I wouldn't trade it for the world.
Good luck.