Published Tuesday, 12 February 2013
The yoga teacher went public with the baby news during a pre-taped spot on her US TV news show Extra, which will air later this week.
She says: "It was a surprise, a wonderful surprise."
Thomas, who wed the actor last year, reveals she took a pregnancy test without telling her husband.
Baldwin adds: "There's a scream I heard, which is a scream normally reserved if there's a spider in the house, not exaggerating. Normally I will be reading a book, we're in the country, it's quiet, some time to really relax and unwind and sometimes once a month you hear 'Ahhh, Alec, Alec!' I come upstairs and there's a spider on the wall.
"I hear that scream again, but it was not a spider."
Thomas recalls: "It was followed by 'I'm pregnant, I'm pregnant!' He runs up the stairs and I'm holding it (test) and showing it to him."
The couple is expecting the baby this summer.
The excited mum-to-be says: "Now, I'm showing, it's getting to be much more, like, 'OMG (oh my god) this is actually gonna happen!' Now we can start the nursery, the everything, the dreaming."