Reply #50: The ebonic-esque dropping of the possessive. Grammatically correct is "baby's mama."

[Shakespeare]

As in, "I have to call my baby('s) mama."

It's a colloquial tendency that goes for almost any possessive within that vernacular. "He stay at Rick house" instead of "He's staying at Rick's house." It's quite common. Drives me nuts, but it's common.

And yeah, it's pejorative, within the vernacular. As somebody already pointed out in the thread, the corresponding positive (when someone is on good terms with their child's mother) is "boo."