The actor explained that it wouldn't be fair to start a family while his primary focus is still on his acting career.
Hamm told The Hollywood Reporter: "It's always on the periphery of the discussion, but we do not have children. You know, it's a tricky balance to strike.
"I have a tremendous amount of respect for my friends who do balance this particular career with raising a kid, 'cause it's a tricky environment to bring them into... So not for us just yet."
Hamm has been dating Westfeldt since 1997, but revealed that they were friends for some time before romance blossomed.
"She was involved with somebody, and I may have been involved as well. Yeah, we were friends for a long time before we officially started dating - and we've never gotten out of the 'dating' thing. We're still in it," he commented.
Hamm previously claimed that he wasn't the first choice to play Don Draper in Mad Men, joking: "I started, literally, on the very, very bottom. I couldn't have had less heat on me."
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