"I've wanted to do this for such a long time, but I kept getting job after job after job," said Eve, who plays former police officer Gwen Cooper in the cult series.
"I thought, if I put this off any more I'm going to be 65. My partner and I decided this year was going to be the year, and we were blessed that it happened so quickly.
"It's the best thing that ever happened to me – we're over the moon."
Eve, who is three and a half months' pregnant, joked about the fact that her character Gwen gave birth in series two - to an alien.
"We went for a scan and my mum was there," she recalled. "And my mum went: 'No, it's not a Nostrovite! It's a real Welsh human!' But of course it crosses my mind – I hope it's not an alien!"
Eve added that motherhood will not make her quit her role in Torchwood.
"I hope we get a new series," she said. "I'd really like to play Gwen as a mother. It's not like I have to really save the world - I just have to sit in make-up!"
Eve's baby is expected to be born this Autumn, she will return as Gwen Cooper in Torchwood: Children of Earth this Summer on BBC One and BBC HD. In other news: John Barrowman stars in the 100th episode of the BBC One sit-com, My Family, tonight at 8:00pm.