Ultrasound image segmentation is hard, as there is lots of noise in the image and surfaces are blurry. Also, the images are usually incomplete due to acoustic shadowing and limited field of view.
That’s why professional baby 3D printing services employ artists, who warp existing generic baby 3D models to the surface patches visible on ultrasound. This way they always get a full and realistic image, with visible details taken from the actual baby. This is not an easy skill to learn and it involves image segmentation (in 3D Slicer) and 3D modeling (Blender, Rhino, etc.), but the results are amazing.