The larger sibling appears as a mangled and bloody zombie, with parts of the skull eaten away. He leaves behind a trail of red creep as he moves, which damages Isaac for a full heart. Unlike Gemini, he will constantly pursue Isaac, not stopping for breath after charging.
Meanwhile, the other half appears as an invulnerable spirit, and is passive unless Isaac gets in his line of sight, in which case, he fires a blood beam similar to Brimstone. He tends to stay floating in the area it spawns in, not moving around much.
Upon death, each half shoots radial blood shots, similar to when Steven is killed.
Much like Eternal Gemini and Eternal Steven, Eternal Blighted Ovum starts with 1 large sibling and 2 small invulnerable ones. The large one acts exactly the same as its normal counterpart but he can slowly regenerate HP and is able to destroy rocks. The other 2 small ones fire blood beams at Isaac. All of them explodes into fast 8-way bullets on death.
If The High Priestess is used on the Blighted Ovum, Mom's foot will stomp on the smaller twin, doing no damage and thus being useless.
They appear in the Wrath of the Lamb DLC for the original game.
The Blighted Ovum no longer appears in the Catacombs since Rebirth.
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. This likely refers to the smaller twin being dead (a spirit).
The large bite on the top of the head is on the opposite side on the artwork and sprite; this is probably unintentional.