Utero

Utero is an alternate floor to. Unlike other alternate floors, Utero does not need to be unlocked, aside from unlocking The Womb. While the theme of Utero overlaps heavily with The Womb, it appears to emphasize gore and disfigurement rather than its counterpart's focus on 'intact' viscera, though the difference in enemy make-up is still more subtle compared to other floors nonetheless. Its walls are slightly different, and the floor contains several new enemies that are bloodier and more deformed, as well as more enemies borrowed from later chapters such as and.

Like The Womb, all damage that is taken on this floor and onwards deducts a full heart from the player's health (unless the player has ), regardless of the source. While in the Utero, a dark yellowish desaturated screen shader and a warping effect will be applied to the screen, making it appear as if the area is submerged in fluid and further distinguishing the floor's appearance from The Womb.

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 Utero is titled Viscera. The term “viscera” refers to the internal organs in the body. shares this same track.
 * The music for Utero is titled Caesarian. A caesarian is a medical procedure where a baby is surgically removed from the mother due to complications during birth.
 * 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.

Binding of Isaac: Rebirth OST