Womb

The Womb is the fourth chapter of the game, unlocked after beating Mom. From the Womb onwards, all damage taken will do a full heart of damage to the player's health (unless the player has ), regardless of the source. Like //, the Womb's second floor has a preset boss that will always be fought: Mom's Heart initially, and It Lives after beating Mom's Heart 11 times.

The Womb has two alternate floors. The, which, unlike alternate floors before it, does not need to be unlocked separately from the Womb, and the.

From this chapter onwards, floors do not include item rooms or shops, meaning that all item buying must be done before the player leaves The Depths/Necropolis/Dank Depths.

Monsters

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Final Boss
Mom's Heart will always appear in the second floor of this chapter or in the last boss room if Curse of the Labyrinth is active. Once It Lives is unlocked, it will appear instead of Mom's Heart.

Music

 * The music for the womb is titled Viscera. The term viscera represents the internal organs in the body. The  shares this same track.
 * The music heard during the fight against Mom's Heart/It Lives is called Ventricide. Ventricide is a word made up of the Latin word for Heart, Ventricle, and the suffix -cide, which denotes the act of killing.
 * This has been confirmed by Edmund when he said it means something like "the killing of the heart". This is in theme with other similar boss tracks such as Matricide, Hericide, and Infanticide.

Trivia

 * The Womb being mentioned in Isaac's last will as a "warm place" is a reference to how the inner human body temperature is quite high.
 * The Womb and both have a slight static filter covering the screen, possibly to represent heat.
 * In the original Binding of Isaac, Lokii and Teratoma were exclusive to Utero.
 * Teratoma was moved back to Utero.