I also agree that this idea would probably be an unwise one. It would encourage unwanted behavior and people would no doubt abuse the function in undesirable ways. Personally, having something like this in the game would just make me feel...rather uncomfortable. This game is mainly for 'children' and although it does harbor a older audience, images and subject matter such as that shouldn't be so blatantly accessible. The function would have to be placed in the character creator and it would be far too easy for people to mess around with it in ways that just are not acceptable.
As others have said above, you can easily tell other players that your character is carrying pups/cubs by adding a tag to your user, or you can place the information in your bio. Also, if you're in a Role Play, and through the story everyone learns that your character has become pregnant, I'm pretty sure they'd remember! Give the players around you a little credit ;u; we don't always forget things so easily!