I'm in a similar situation but with the genders reversed -- I started dating my girlfriend, Gia, a year and half ago. I've been physically intimate with her husband, Eric, and consider him a good friend, but don't have the same sort of relationship with him that I have with Gia. About a year ago, they started making serious plans to have a child. Five months ago, Gia became pregnant.
Obviously, much of this will be very different for you since you're with Dave, not Amy... so, this is just me rambling in the hope that something in here will be pertinent or helpful.
The pregnancy has changed a lot of things already. Gia's had a rough pregnancy so far. Nothing TOO scary, thankfully, but she's been very nauseous, achey, and exhausted on a frequent basis. Less frequently she'll have more acute problems like gastrointestinal issues that really knock the wind out of her sails. I haven't been sexually intimate with either of them since she conceived. Her physical issues related to the pregnancy are a big part of it, but also she just has wanted more space. It stings, but I recognize that this is a transititional period and so I'm not pushing.
One surprising thing, for both of us, was that Gia felt her internal world entirely rearrange after she conceived. She had had plans and ideas and goals for her future that didn't have to do with the baby and it all sort of went out the window at first... those things are coming back now, but not necessarily in the same order or with the same force. It's all about the baby right now. My place in her life... what we might be building together... falls into the category of those things that she knows are important to her but that she can't actively plan for right now. So we're just riding it out.
I'm preparing to fall deeply in love with the child-to-be. How could I not, since it will be a part of people I love? I see it as being a big change that will come in my life, and a big responsibility... I've actually considered getting a tattoo representing the child's name when he/she arrives. It would be my first tattoo. That might give you a sense of how deeply committed I feel to the idea of being a part of this child's life, helping him/her grow and thrive and learn and live and love.
Gia has said that she sees me as an Aunt figure for the child to be, which I'm comfortable with. The big question for me is -- would I be willing to be more? Would I be willing to be a co-parent? I don't know if the two of them would even want that, but I can't decide whether or not to offer unless I know if I would want to follow through. How would I find the time? What would our families think (we're all soooort of out about poly right now)? Would it even be possibly, logistically, since I don't live with them?
The biggest lesson for me has been not to freak out trying to get answers... from myself or from her/them... about what shape the future is going to take. It's just unknowable. Things are completely different now than they were five months ago, and they'll be completely different in another five months. Children are change, is what I've been learning so far (or, at least, pregnancy is). I'm more excited than I am scared, more in love than I am worried about getting burned.