Blastocyst Portrait
Character Information
Character Type Boss
Floors of


The Womb
First Stage Health 190
Second Stage Health 75
Third Stage Health 30
Fourth Stage Health 12
Items Dropped Boss Pool

Blastocyst is an enemy in The Binding of Isaac: Rebirth that is present as a Boss on The Womb and the Utero Floors and also during Boss Rush. It is present as a Monster on later Floors. Its little embryos, however, spawn as early as The Basement.


Blastocyst has several stages that it goes through. The stages are as follows:

  1. Blastocyst will be one large entity and will occasionally shoot tears in an asterisk pattern as it hops randomly around the room.
  2. Once killed, Blastocyst will split into two smaller entities that will occasionally shoot tears in an asterisk pattern as they hop randomly around the room.
  3. Once those smaller ones are killed, even smaller Blastocysts will spawn, shooting 4 tears in either a plus pattern or an "X" pattern.
  4. Once those small ones are killed, an Embryo will spawn.


Piercing tears or explosives are extremely effective against Blastocyst. Also, pushing Blastocyst into spikes can be rewarding, for it won't split (same when it takes extreme damage).

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