Anna I've felt the same way as you and decided to take the openly honest approach with my friends, including first time pregnant friends. I think the key is to explain that motherhood can be very difficult and to describe the difficulties but also to make sure you explain that in spite of that it is worth it.

Also I think it's good to be honest about our feelings so that more women will be able to talk about their honest feelings without fear of judgement and to receive support from each other. One of my friends only told me after I had my DD that she had suffered PND alone and really struggled with her DS. I wish I'd known that at the time so that I could have been there for her.