Stringfield, 41, gave birth to the baby in December.
"They've had a baby, and how they kept it a secret nobody knows," a source told the Post.
"Steve's very private," the insider said. "They are thrilled. They worked hard to have the baby."
The child's gender and name were not reported.
Stringfield and Martin married in 2007. They live in Los Angeles.
LGBT community has 'bullied the American people': Bachmann
Handler slams Piers Morgan: 'You're a terrible interviewer'