Miller found being a mother very difficult at first because she had such a hard time giving birth to her daughter Marlowe. She went through 27 hours af labour and then had to have an emergency caesarean.
She said: "My birth went all wrong. I had my heart set on this natural water birth and I ended up, after 27 hours, having an emergency C-section.
"I spent so much time thinking about the birth and not realising that you take a baby home at the end of it.
"It's a shock, of course it is, and some people find it really easy, but for me I found it really hard."
She has recently said, however that being a mum has helped her in her most recent movie role in American Sniper alongside her on screen husband Bradley Cooper.
She added to Time Out magazine: "I think being a mother and playing a mother, yes, I understand motherhood in a way I just couldn't before.
"I had a huge amount of empathy for Taya. To be going through motherhood on her own, not knowing whether her husband would survive every day, I can't imagine how difficult that would be.
"Especially because that experience of motherhood for me was really complicated."
Tagged in Sienna Miller Bradley Cooper Tom Sturridge