For me it was an emotional thing. Not necessarily an event. With number one I felt like it was an 'it' in an ambiguous kind of way. I loved my babe so much, talked to him all the time, prayed for him, but just had no concept of what 'feeling like a mother' was, so he was still an 'it' until sometime after he was born. Actually, my very first thoughts the first time I held him (moments after he emerged from my body) were, "What does this mean?" LOL Well, it was a slow, sweet falling in love.
With my second I didn't feel like he was my son (even though we had found out his gender this time--partially because I felt so disconnected and thought it would help) until I had him in my arms. Somehow he remained an 'it' the entire pregnancy. I had some fears about if I would love him as much as my first son (my first has special needs, so I was actually afraid I would not love a 'normal' child the same). As soon as he was born I knew I loved him deeply, and I felt connected to him. It was immediate.
With this pregnancy, it was through an emotional journey, not an event, that I came to feel deeply connected to this boy already. I feel like I already know him, already know how to parent him (we'll see, eh?). So, who knows how these things happen. I think it is different for each mother, and however it happens is okay. I think the first time is a little different too, simply because experiencing pregnancy has so many thrills an preparing your lives for a baby can be such a fun task. Anyway, the second trimester is something to look forward too. It does make things more real when people around you can readily recognize that you are pregnant.