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Boss Blastocyst.png
Base HP
Large: 190
Medium: 75
Small: 30
Embryo: 12
Stage HP
Found In
Stage womb.pngStage utero.pngSuper Secret Room Icon.png
Dropped Items
Boss Boss  

Blastocyst is a boss that spawns in The Womb / Utero. It is a large blob with a small Embryo inside.

Behavior[edit | edit source]

  • Hops around the room randomly and shoots 6 tears in all directions.
  • Performs a long-range jump to the player's position, shooting 8 tears in all directions upon landing.
  • Once enough health is depleted, it will split into two medium versions of itself, which shoots 6 tears in all directions.
  • These two medium blastocysts each split into two smaller forms, which shoot smaller tears in 4 directions.
  • When all the parts of the blastocyst are defeated, they will become a bouncing Embryo that does contact damage.

Blastocyst is perhaps the least complicated boss in this chapter, having essentially one attack that gets weaker for each split. Any sort of piercing tears or items such as Technology 2 Technology 2 or Brimstone Brimstone can defeat this boss extremely quickly.

Gallery[edit | edit source]

Trivia[edit | edit source]

  • An embryo is called a blastocyst when the differentiation of cells within a morula finally begins, usually 4-5 days after the fertilization of an embryo.
  • It is very common for the second level of the boss to spawn as normal enemies in rooms. On even later floors like Sheol, the full boss can even spawn as a common enemy.
  • If Blastocyst is killed on spikes, none of the smaller versions will spawn. This can also apply if a very large amount of damage is dealt very quickly to any of the phases.
    • This can also happen if Blastocyst is killed while still having a debuff that actively harms it (e.g. poison, fire).
  • Upon killing a blastocyst, it seems as though, for a split second, an audio clip of someone's voice can be heard. To better hear it, listen with headphones without the music.
  • Added in Afterbirth † Prior to an update, an unfair room existed in The Womb consisting of 8 Blastocysts as result of unintentionally placing full sized ones rather than small ones.
  • As Blastocyst divides, its positive facial expressions will become less happy until it becomes a bouncing embryo which has no smile at all.

Bosses Boss Monstro.png
Chapter 1 Blighted OvumDingleThe Duke of FliesFamineFistulaGeminiGurdy Jr.GurglingsThe HauntLarry Jr.MonstroPinStevenWidow
Added in Afterbirth DangleLittle HornRag ManTurdlings
Chapter 2 C.H.A.D.ChubDark OneGurdyGurdy Jr.The HollowThe HuskMega FattyMega MawPeepPestilencePolycephalusCarrion QueenThe Wretched
Added in Afterbirth The ForsakenThe FrailThe Stain
Added in Afterbirth † Big HornRag Mega
Chapter 3 The AdversaryThe BloatThe CageCarrion QueenThe GateGishLokiMask of InfamyMonstro IIWarThe WretchedMom
Added in Afterbirth Brownie
Added in Afterbirth † Sisters Vis
Chapter 4 BlastocystThe BloatConquestDaddy Long LegsDeathIt LivesLokiiMama GurdyMom's HeartMr. FredScolexTeratomaTriachnid
Added in Afterbirth Blue Womb Added in Afterbirth Hush
Chapter 5 The FallenIsaacSatan
Chapter 6 ???The LambMega Satan
Added in Afterbirth Greed Mode Added in Afterbirth Ultra Greed
Added in Afterbirth † Ultra Greedier
Added in Afterbirth † The Void Added in Afterbirth † Delirium
Chapters 1-4 The FallenThe Headless Horseman

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