The brunette star already has a seven-year-old daughter, Lily, with ex-boyfriend actor Michael Sheen but now wants a child with her director husband Len Wiseman.
However, Kate says she needs to advance her career before she starts trying for another sprog.
She told OK! magazine: "I want another child but I'm not ready to stop my career and look after a baby for a year. I need to work a few more years before I get to that point, but I could always change my mind."
Beckinsale has also revealed she wants Click co-star Adam Sandler to be her new babysitter.
She thinks he would be perfect to look after Lily because he bonded with her so well on set.
She said: "Lily was enchanted by Adam. He has such a wicked way to make children laugh. I wish I could hire him in the afternoons!"