The Womb is the fourth environment of the game. Utero can replace The Womb. The player will enter it after killing Mom, but only after they have beaten her once before. It is possible to access the Womb before even killing Mom once, by digging down from Depths II using the We Need To Go Deeper! item. The enemies in this enviroment are mainly worms and other medical things, and their behaviour often involves disappearing and reappearing across the room (i.e. with the Baby's teleporting or the burrowing behaviour of Para-Bites, Lumps and Fred ).

You can also access The Womb without killing Mom by being teleported in the I AM ERROR room by using a Telepills pill from the Depths 2 level.

In the Womb levels, enemy hits will do a whole heart of damage instead of a half.

Many rooms in this environment have boss monsters as "regular" monsters - the boss health meter will appear, but no special item is granted upon defeat. Items as if you were in a "regular" room would drop. Boss monsters selected include Basement and Cave bosses, as well as Sins (can be replaced by a super version per room) and Loki. There can be two of the same Sins in the room, and, on rare occasions two sins and a basement or cave boss. Gemini can appear in pairs and there can be multiple Monstros in the same room.

The Womb has no Shops or Treasure Rooms, but it does sometimes have Arcades. The Womb can also contain a Challenge Room as well as a Boss Challenge Room. Challenge Rooms in The Womb contain bosses from The Basement and The Caves.

MonstersEdit

• Brain
• Dople (upgraded version is extremely rare)
• Eye (upgraded version is rare)
• Fly
• Fred
• Host
• Leech
• Lump
• Mom Brain
• Para-Bite
• Poky
• Globin
• Guts
• Mr. Maw
• Vis

Boss monsters in normal roomsEdit

• Blastocyst (multiple in the same room, all but the largest version)
• Chub (can appear with 2 Larry jr.s)
• The Duke of Flies (rare, 2 in the same room)
• Fistula (whole)
• Gemini (2 in the same room)
• Larry Jr. (1 to 4 in the same room, sometimes with other enemies or Chub)
• Loki
• Monstro (can be up to 4 in the same room)
• Peep

BossesEdit

• Scolex
• Blastocyst
• Death (once unlocked)
• The Headless Horseman (once unlocked)
• Daddy Long Legs
• Triachnid (once unlocked)
• The Bloat
• Mom's Heart/It Lives (Always encountered in Womb II/XL. It Lives replaces Mom's Heart after the game has been successfully completed nine times)

Differences from higher worldsEdit

• Enemies do a whole heart of damage instead of half.
• Shop and Treasure rooms are disabled.
• Bosses from higher worlds now come in the form as normal enemies.
• Blood donation machines take away a whole heart instead of half.
• Two of the seven deadly sins can be encountered in one room multiple times.
• Entering a curse room takes away a whole heart, instead of half.

GlitchesEdit

• If there's a Devil Room after defeating Mom's Heart for the ninth time (before unlocking Sheol), entering it and dropping down the trap door sends you to Sheol. If you defeat Satan, it unlocks the item The Poop, and ??? before defeating It Lives.
• Sometimes there can be an Angel Room instead (changes nothing).