Just got Charm of the Vampire, which is supposed to heal half a heart every few kills. Sounds amazing, but.. I made the mistake of picking up Dead Cat earlier. I have only one heart now.
edit: Fighting Mom with one heart is not funny. Beat her, but only after losing three lives.
edit 2: Lost all lives except the final one. Against all odds, I managed to clear a huge, crowded room except one of those mole-like enemies. I could still hear him, so I carefully searched the room. He appeared right below me. Unbelievable.