Troubles can come when hearts are bigger more than 50ds. I've learned two ways to obtain acceptable hearts, I've numbered pictures and hope it could help some of you. First way (pictures from 1. to 9.): start as a normal ring, then make a loop in the center but follow Jane's hint, pulling plenty of thread and wrap it around the little finger as though you are tatting a chain.
Second method is tatting the heart with the mocking ring techique (picture from a. to f.) so start pulling a loop for the mock ring, then tat half heart and pull the second loop. Before closing the SCMR, put the shuttle (or a loop from the shuttle) through the first loop, then pull the dimple loop to close the first loop, then pull the core thread to close the dimple, after then close the SCMR.