Your Initial Reaction
When a woman learns that she has a child growing inside her, it is a profound thing. When a man learns that there is a child growing inside someone else—that's a different thing. Your reaction will be powerful and complex. Most guys feel a mix of elation, apprehension, and shock that their world is changing (remember this moment: it's the closest you'll ever come to the hormonal emotions of a pregnant woman).
In the back of your mind you may also have a trapped, panicky feeling of a sudden responsibility you can't get away from (don't feel guilty—everyone feels this: if you didn't, you would be in denial). Remember it's just a temporary reaction: a week later, you most likely will feel very differently. But still apprehensive.
Now, the way she wants you to react is another matter, and she will be looking into your eyes waiting for you to light up with some kind of pregnancy glow and say, in a deep, Barry White-like voice: "Baby, you just made me the happiest man in the world, and I'm gonna take care of you forever."
So, when instead of turning into Barry White you freeze and squeak, "Are you sure?"—she might be disappointed.
On the other hand, if you have been really trying for a long time, you will probably be relieved and hopeful. And apprehensive. That's okay too. Just take it as it comes.
It's not about being perfect. It's about being involved, my friend.