In an interview posted today on CNN.com, Berry, showing off a tiny baby bump, said, "This has been the biggest surprise of my life, to tell you the truth. Thought I was kind of past the point where this could be a reality for me. So it's been a big surprise and the most wonderful."
The 46-year-old actress added, "I feel fantastic."
Berry also has a daughter, Nahla, 5, with a former boyfriend, model Gabriel Aubry.
The actress said last year that her daughter, Nahla, had been hoping for a younger sibling, and the 4-year-old is finally getting her wish.
"[Nahla] says, 'God, please bring me a bunk bed and a baby sister,'" she said last fall. "I say, 'I can do one of those things, I know for sure. The other one, we've got to keep praying on.'"
TMZ reported that Berry and Martinez, 47, are expecting a baby boy, but she told CNN she did not know what she was having.
Berry recently teamed up with designer Michael Kors to fight hunger through the United Nations World Food Programme.
"Especially now, being a pregnant woman and already having a child … it's so important what happens to the baby while they're in utero. And the first 1,000 days is fundamental," Berry told CNN.