I drew half of a rib cage on a regular piece of paper. I cut out the pieces to use as stencils and traced them twice (the second time in reverse) with a sharpie on some white felt.
I sketched a baby skeleton on another sheet of paper. I found plenty of inspiration online.
You can buy pregnant skeleton shirts or pregnant skeleton iron ons from various websites.
I used the window as a light box to trace the design on to some white felt. I ended up cutting the baby skeleton out in one big piece and filling in the negative spaces with the black sharpie.
I was going to use liquid stitch to attach the felt to my black shirt, but I couldn't find it. I went with glue gun instead. It held up for two days of all day wearing (over a long sleeve shirt). I don't know how it would do in the wash especially since I used a regular sharpie and not one designed to withstand laundering.