Maybe that's because she's got the cutest baby around - Marlowe!
Sienna's buddy slash photographer took the most amazing picture of these two.
Here's what the photographer wrote:
"Mummy Marlowe Love @siennaprivate."
We love the whole hippie thing that Sienna has going on in this picture.
But what is going on with Marlowe's hair!
It's all over the place!
And yet she's still adorable!!
Now that's amazing!!!
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[Image via Instagram.]