"Look. I'm 37 years old," the Revolutionary Road star told Time Out. "We have one life. I don't want to spend my time thinking about the size of my a**e.
"I want to be as healthy as I can be, and I want to have as much fun as I can have. I want to be around for my children. That's it. Those are the priorities. Not getting a flat stomach.
"Don't get me wrong. I like staying fit and healthy. It's all part of a healthy attitude to life."
The Hollywood star, who said she has been "lucky" and not had any major problems in her pregnancy, revealed that she wasn't going to rush into shedding her post-baby weight.
"How does anyone have the time?" said Kate. "Also, let's be real here. Everyone is different. Springing back into shape after a baby — that's not really something that's ever happened particularly swiftly for me. That's just me."