There was an episode that came up that revealed the truth (I am a new fan, so it's new to me):

Eric Cartman is an OC. His mother was the other woman.

She is the very picture of the worst kind of other woman--seems to throw herself at any man, even if he's married.

Cartman's mother ignores him as much as possible. He is foul-mouthed, a master manipulator, and allowed to speak ill of jewish people with no repercussions. She also doesn't seem to care how he is treated by his babysitters.

For any women who wonder how the other woman treats kids--any kids...yours, hers....that is how.

This, women, is why you should fight to keep your kids away from the OW if at all possible. She doesn't give two *&$ts about your kids.

His Mom turns out to be a hermaphrodite who inseminated someone else, so she is actually his Dad but who his Mom is is still unknown.

The show is crazy.

Which episode is this?

The last episode I think I saw is where Scott Tenorman's dad was said to be his...though I think I only saw part of that one.

Ah, ok I haven't seen those. I thought you were referring to the early series storyline. Please excuse the language of the episode title.

http://en.wikipedia.org/wiki/Cartman%27s_Mom_Is_Still_a_Dirty_Slut