Banks and her husband Max Handelman welcomed son Felix in March 2011 via a gestational surrogate.
In an interview with Allure, Banks said her 30 Rock co-star Alec Baldwin sensed that she was having a baby even though she wasn't actually pregnant.
"Alec Baldwin asked me straight up [if I was having a child]," Banks laughed. "I had made a comment about wearing a fake belly, something innocuous, and he looked me straight in the eye and asked me, 'Are you pregnant right now?'
"And I blanched totally. Because of course, technically, no I wasn't. But then, technically, yes, in a way I was! And it was very weird."
She went on to quip: "I thought Alec was reading my mind or something. We weren't telling anyone then, so I said 'No'. I thought, 'Is that a lie?' Because it wasn't a lie, and yet it was a lie."
Banks also described the process of using a gestational surrogate to have a child.
"It's essentially a test tube baby - it's just the test tube is another woman," she explained. "That you can grow a baby outside of yourself - it's amazing."
Banks co-stars with Jennifer Lopez and Cameron Diaz in What to Expect When You're Expecting, now playing in the US and opening on May 25 in the UK.
Watch a trailer for What to Expect When You're Expecting below: