I second LR's suggestion to read this thread: http://www.polyamory.com/forum/showthread.php?t=1127
But now, and for the last 5 years, it's just been us.
You've become interdependent. (Not co-dependent). Your identities, in a sense, have merged more than when you were first together. Now one of your "pair" is branching off. The energy is flowing outward, where it's been inward for a long time.
That can be scary. Your nest together, seemingly where you find your security and routine - your family, is shifting. You know, I've never had kids, but I suspect a first pregnancy can bring similar feelings. It doesn't sound like you're jealous, really. But that your world is changing and that's scary...