(US, slang) Mother of child in common, particularly unmarried.
She's not his girlfriend now, but she's one of his baby mamas.
Origin of baby-mama
African American Vernacular English (General American would be baby's mama), 1990s, popularized 2000s. Possibly from or influenced by same term in Jamaican English, from Jamaican Creole baby-mother (“pregnant woman”) (1966), alternatively due simply to grammatical similarities between AAVE and Jamaican Creole.