Wahlberg, 36, and longterm partner Durham, 29, have yet to go public with their news - but the Boogie Nights star has reportedly told friends he is expecting a son, due in September.
A friend close to the star tells U.S. magazine OK!, "Mark is thrilled to be having another baby. He is an amazing dad, and his two children are his world.
"He always used to joke about wanting at least a dozen kids, but it looks like he wasn't joking after all."
Wahlberg and Durham - who have been dating since 2001 - are already parents to daughter Ella Rae, four, and two-year-old son Michael.
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