This heart container will be filled the first time the trinket is picked up.
Notes[]
If the only heart container Isaac has is the one given by Mother's Kiss, the trinket will be destroyed if the heart container is traded/consumed (such as by taking a devil devil deal)
If Isaac drops the trinket while he only has one heart container for health, he will have no health and die instantly.
This also means that using 0 - The Fool? card will do the same, even if he has soul hearts.
The extra red heart container can exceed the maximum coin heart container limit of Keeper and Tainted Keeper, allowing them to obtain more than 3 or 2 coin hearts, respectively.
Using The Fool? card as one of these characters while holding this item will kill you instantly, as it drops the trinket, removing the last half soul heart that you normally would remain with.
The Forgotten will always get a Bone Heart regardless of whether the body or the soul picked up the trinket.
While playing as characters who cannot have red HP, if Mother's Kiss is picked up while having 12 Soul/Black Hearts, Mother's Kiss will not trigger until damage is taken. However, if only half a heart of damage is taken, Mother's Kiss will immediately restore the health back to a full 12 but believe its effect failed due to not restoring a full heart's worth, allowing it to trigger again and again until a full heart of damage is taken, after which it will cease taking effect. If combined with The Wafer, this makes any of the aforementioned 6 characters virtually invincible as long as their health is not reduced in any way through Devil Deals, Health Down pills, etc.[1]
Bug!
Dropping Mom's Box will not remove the second heart container until the trinket is dropped.
Bug!
Dying and respawning with Dead Cat will respawn you with only 1 heart container, meaning you will instantly die if you drop the trinket.