The Alfie star and actor partner Tom Sturridge became parents to daughter Marlowe in July (12), but the pair has kept largely quiet about the new arrival since the birth.
Miller has now opened up about motherhood in a new interview with Harper's Bazaar magazine, revealing she was wary of taking on hired help while she figured out how to tend to the newborn.
She explains, "It's just been the three of us, which was amazing, and exhausting. We are going to get a nanny, but it felt important to first learn for myself who I was as a parent."
Miller is convinced becoming a mother has changed her for the better, insisting it is the "greatest thing" she has ever experienced.
She adds, "It's still the most surreal thing, that you can create... that I grew this life inside my stomach, her eyeballs, everything - every little fibre.
"And I'd expected that she'd be this extension of me and I'd instantly understand who she was because she'd come from me, and then you realise that they are their own people entirely. Very connected to you, of course. But they're their own person... It's the best, the greatest thing in the world... I would do that day (of the birth) a million times again. I would do that day, every day. I loved it."
The 30 year old reveals she is now at ease with her figure after giving birth: "My body's a completely different thing to me, it's not mine - all the attachment to its flaws or any aesthetic attachment is gone. You understand what breasts are for; and I have such enormous respect for my body because of what it can do."