How can I make t-shirts with baby skeletons in it and make sure I am not violating any copyright?

Copyright only protects the actual expression of the idea not the idea. For example photographs of say the Empire State Building are on postcards and are copyrighted to the photographer that took that photo. You cant sell postcards with the exact photo made from the same negative but you can take your own photo of the ESB and sell it as it will be copyrighted as your own expression.

Be creative and design your own version of the skeleton - do not copy someone else. That is the secret to copyright - your concept is your property. Another person's concept is their property. The first person to have a skeleton drawing on a t-shirt does owns nothing more than "that version" of a skeleton.

One can copyright an expression but not the underlying idea. So someone can copyright a stylized skeleton on the bottom half of a t-shirt but not the overall idea of a fetus' skeleton placed over the part of a t-shirt that would cover a woman's stomach. Your skeleton must be different enough that it does not duplicate another's skeleton.

A good way to avoid copyright violation is to make sure that whoever is designing/drawing a design is not looking at or copying from a design/drawing by someone else. Create from your own mind.