For one, it's humbling and exciting and frightening all at the same time to realize your baby really is alive. I mean I can't feel the baby move yet and my amount of showing is still pretty insignificant - I don't feel all that different from the week after Thanksgiving. After the woes of the first trimester pass you're left with this eerie feeling of normalcy, like - I don't feel pregnant suddenly. It's scary, feels like you've just been fooling everybody, but to hear the heartbeat is a rush of reality that the baby is there and is growing.
On the other hand, it's mind boggling to think how big the baby is now - 3 inches long, that's like half a banana (or a whole little banana - the kind people grow in their backyards down here) or a big shrimp or a decent sized apple wedge or a clothes pin. That's way too big to be Barbie's baby. Still small, I know, but I go back to the weird (fleeting) not-feeling-pregnant feelings, it's like if you had a tumor THAT big in you, could you not be aware of it? I guess it depends.
Caleb eating half a banana - for size reference...