After the initial feeling of bliss/fear, things will settle, but only for a moment. You'll start to notice your lady develop mood swings that you've never seen before. Sure, you've both been moody before, and you have your normal ways of dealing with those types of things as a couple. I just want you to know that you need to forget everything you know, and develop what I like to call "Emotional Flip."
One afternoon, about 6 weeks into pregnancy, my wife and I were grocery shopping. She immediately started to hate everyone in the grocery store. I kind of laughed to myself because she was tearing through the aisles and commenting toward people who were socially inept, like people hogging the aisle. "Yeah you tell 'em," I thought. Then I made a comment about someone too, joining in on the game that I so love. She fell silent. She was silent all the way home, silent through unpacking groceries. "Are you mad at me?" I foolishly asked. "NO! ARE YOU MAD AT ME?! BECAUSE IT SEEMS THAT WAY! YOU WERE A JERK THE WHOLE TIME IN THE GROCERY STORE!" She stormed off, leaving me with my jaw agape. This was my first real hormone swing. Needless to say, I did not handle it well. I yelled back, like I normally would when being attacked like that. It did not help.
I had been told time and time again that my wife's hormones would shoot through the roof unexpectedly. I just never thought that I couldn't handle it. I reacted hurt and angry which only made her more hurt and angry. There was a misunderstanding because, you see, she is living with a succubus in her ever-expanding uturus. This beautiful, little creature is tossing her body and emotions all over the place. You just can't see it. If this were The Exorcist, she would be climbing the walls and you would say, "oh, I understand, I see how terrible that must feel." But you can't see it, and you still have to understand.
Emotional Flip. This is what you need to acquire for when you are suddenly the subject of her swing. Sometimes you'll ask her what you think is a simple question, and she will bite your head off with a sharp, "I DON'T KNOW!" And you'll want to say, "What the fuck? God!" But instead you need to stop. Count to 10 (I mean it). Remember The Exorcist. Do the Emotional Flip. And say, "OK, no problem, I'll figure it out." You need to immediately realize that none of this is personal. You are the one she is around the most, so you're going to have to deal with these moments more than anyone. It's as simple as that. Sometimes you'll be tired too, and you'll snap back, you're human. Just make sure you apologize, even if you aren't wrong. It's a tough pill to swallow at first, but if you can stop before lashing back and remember what SHE is going through, you'll do just fine.
Be her rock. That doesn't just mean carrying every bag of groceries, or doing every chore in the house (both of which you should try to do as much as possible). It means emotionally too. She needs that more than ever, especially in the beginning. Emotional Flip is the difference between a snuggle and sleeping on the couch.