Adds half of one white heart, lost before normal hearts, but after soul hearts. When using Blood Donation machine, the white heart is lost first. Changes to a full normal heart container after the floor is completed, or if a second eternal heart is acquired before completing the floor. May spawn instead of a regular heart. (Using the Forget Me Now Treasure will automatically complete an eternal heart as if you finished a floor.)
--- Taken from the Binding of Issac: Wrath of the Lamb wikia.