Someone posted this on my month board, and I found it interesting. I thought I would share.
http://www.thevisualmd.com/media_gallery_slice.php?idu=12775&idc=1138
So basically at 12 weeks the baby is the size of a plum, and your uterus is the size of a grapefruit. (Obv the "empty" space in the uterus is filled with fluid.)
If you're small/petit/slender I can imagine could be showing slightly at 12 weeks. Especially if you're not very tall and there isn't much room for the uterus/baby to go.