Blighted Ovum

Blighted Ovum is a boss that can appear in the Cellar.

Behavior

Blighted Ovum starts separated into two parts.

Suture, now a ghost, is invincible and slowly floats towards Isaac, firing a horizontal Brimstone shot whenever Isaac is to his left or right. Suture cannot be killed and will die once Contusion does.
Contusion constantly chases Isaac while leaving behind a trail of Red Creep, and he periodically charges and gains speed. Unlike Gemini, he will not stop for a breath but slows down to his regular speed.

Blighted Ovum as one of Delirium's forms.
Sketch of what appears to be an older concept of Blighted Ovum, along with other bosses that were to be added in the original The Binding of Isaac's Wrath of the Lamb DLC. Note that the behaviors mentioned in the concept art were never implemented in the original game onwards.

Suture can be killed by Little Horn.
If encountered in a Double Trouble boss fight, Suture's Brimstone laser can damage the other boss.

Blighted Ovum was added in the Wrath of the Lamb DLC for the original The Binding of Isaac.
In the original game, Blighted Ovum could also be encountered in the Catacombs.
They are the posthumous version of Gemini.
In medicine, a blighted ovum is a pregnancy in which the early pregnancy appears normal on an ultrasound scan, but later no visible embryo develops.
Contusion has skin on his body in the boss art, but he doesn't in-game.
Blighted Ovum is the only boss in chapter 1 that is a posthumous version of another boss.
- Gemini also shows up in chapter 1, which makes it one of only four bosses to have its posthumous version in the same chapter, the other cases being Polycephalus and Chub having The Stain and Carrion Queen, respectively, in chapter 2; and the Tuff Twins having The Shell in Chapter 2.5.
In Wrath of The Lamb, when Blighted Ovum was killed, Contusion would shoot 8 extremely fast tears in the cardinal and ordinal directions, and after a short delay, would be followed up by Suture, who would explode similarly.
In the Bestiary, Suture is referred to as "The Blighted Ovum Baby".
Blighted Ovum, Big Horn, Triachnid, Isaac, and ??? are the only bosses whose portraits were completely changed in Repentance (beyond simply adding more detail). Of these, only Triachnid, Isaac, and ??? didn't receive new sprites.
In Rebirth, the older boss portrait had Suture smiling in it, which was the 2nd time he had a smile.