Sienna Miller froze some of her eggs after she turned 40.

The Layer Cake actress, who turned 40 in December, revealed to Elle magazine she felt pressure to give her nine-year-old daughter Marlowe a sibling during her thirties but her biology has been "incredibly cruel". Since she took the step to freeze some of her eggs, the British actress has felt the "existential threat" lessen.

"(I felt) pressure (about) kids, and should I have more, and why haven't I, and all of that, which is a really loud noise," she explained. "Biology is incredibly cruel on women in that decade - that's the headline, or it certainly was for me. Then I got to 40 and I froze some eggs. Having been really focused on the need to have another baby, I'm just like, if it happens, it happens. That kind of existential threat has dissipated."

Sienna also shared that she likes "the tranquillity" of her relaxed routine in New York with Marlowe and feels more in control of her life.

"There's just much less anxiety, for no specific reason, other than maybe turning 40 and surrendering to being on some sort of life journey that is completely irrelevant," she said. "I think I know what matters to me: my friendships, my child and, ultimately, trying to be happy. That really is enough, with bursts of intensity and creativity."

Sienna shares Marlowe with her ex-boyfriend Tom Sturridge.