Arnold Schwarzenegger is ''really looking forward'' to becoming a grandfather.

The 'Terminator' star will welcome a grandchild into the world when his daughter Katherine Schwarzenegger gives birth to her first child with husband Chris Pratt, and Arnold has said he can't wait to ''play around'' with the impending arrival.

Arnold also confirmed Katherine is due ''this summer'', but insisted he doesn't know ''when exactly''.

Speaking during an appearance on 'The Tonight Show starring Jimmy Fallon', he said: ''Yeah, can you believe that? I mean, that is really exciting news that Katherine now, you know be pregnant and have a baby - I don't know when exactly. Sometime this summer. And, inevitably, I'm really looking forward to playing around with whatever it is, she or he, and have some fun.''

Katherine was first rumoured to be pregnant last month, and sources at the time claimed she and Chris - who already has seven-year-old son Jack with his ex-wife Anna Faris - had kept the news private at first in order to ''enjoy this time'' for themselves.

An insider said: ''Chris and Katherine are so happy. They wanted to enjoy this time privately for as long as possible, but now that Katherine is showing more, they're excited to share the news with everyone.''

The news came after the 'Guardians of the Galaxy' star, 40, had said he was sure Katherine, 30, would make a ''great mum''.

He gushed: ''She has changed my life for the better in so many ways. My heart, my soul, my son I feel are all so safe with her. She's a great step mum. She's, God willing, going to be a great mum one day. She's got great parents, great siblings, she fills in all my many deficits. My nickname on these press tours is Yard Sale because I'm such a scatterbrain. And she doesn't mind helping me with that stuff. It's a good match.''