Immaculate Conception

Effects

Upon picking up enough heart pickups, Isaac gains a permanent angelic familiar and generates a Soul Heart drop. Possible familiars include:
All five familiars will spawn in a random order, each after collecting 15 hearts.
After all familiars have been spawned, a Soul Heart will still drop every 15 hearts collected.
Isaac will look progressively more pregnant as he collects heart pickups.

All types of hearts except Gold Hearts and Blended Hearts count.
Items that directly add health such as Yum Heart, Book of Revelations, Satanic Bible, and The Nail do not count towards spawning a familiar.
Only one of each type of familiar can be spawned.
(The Lost) The Lost can pick up Soul Hearts and Black Hearts despite having no health and thus can activate Immaculate Conception.
(Keeper) Keeper cannot encounter hearts, thus the item is useless for him.
As the short-lived Half Red Hearts dropped by (Tainted Magdalene) Tainted Magdalene also count, it is an excellent choice for her since she will constantly be picking up hearts and gain all five familiars quickly.
The familiars spawned by Immaculate Conception will still spawn even if Isaac has already obtained the familiar through their respective pedestal item.