The actress contributed to a book of feminist essays called writing a piece called: "The Weaker Sex" and she didn't hold back her disapproval of Kate's post-baby look after giving birth to Princess Charlotte in 2015.
The 33-year-old actress gave birth to her daughter, Edie, the day before Princess Charlotte was born.
And she was NOT happy with how Kate presented herself to the world hours after giving birth.
"We stand and watch the TV screen. She was out of hospital seven hours later with her face made up and high heels on. The face the world wants to see. Hide. Hide our pain, our bodies splitting, our breasts leaking, our hormones raging. Look beautiful, look stylish, don't show your battleground, Kate. Seven hours after your fight with life and death, seven hours after your body breaks open, and bloody, screaming life comes out. Don't show. Don't tell. Stand there with your girl and be shot by a pack of male photographers."
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After looking at the pictures and videos showing the Duchess smiling and holding Charlotte on the hospital steps, Keira was struck by the stark contrast to what she was experiencing after becoming a mother, writing:
"My vagina split. You came out with your eyes open. Arms up in the air. Screaming. They put you on to me, covered in blood, vernix, your head misshapen from the birth canal. Pulsating, gasping, screaming. You latched on to my breast immediately, hungrily, I remember the pain. The mouth clenched tight around my nipple, light sucking on and sucking out. I remember the s—, the vomit, the blood, the stitches. I remember my battleground. Your battleground and life pulsating. Surviving. And I am the weaker sex?"
Kate has looked ultra glam after the birth of all three of her children.
And of course it's (hopefully) her decision to appear like that. In fact, it's really a mother's choice how she wants to look or behave after giving birth.
Rather than attack Kate, Keira's essay is intended to combat the pressure on new mothers and women in general to look or appear a certain way, as well as how we deal with our jobs.
She writes: "I turn up on time, word perfect, with ideas and an opinion. I am up with you [her daughter] all night if you need me. Sometimes I cry I'm so tired. Up with you all night and work all day... My male colleagues can be late, can not know their lines. They can shout and scream and throw things. They can turn up drunk or not turn up at all. They don't see their children. They're working. They need to concentrate."
She explains women must "be pretty. Stand there... Be nice, be supportive, be pretty but not too pretty, be thin but not too thin, be sexy but not too sexy. Be successful but not too successful. Wear these clothes, look this way, buy this stuff."
She ends the essay finishing: "I work with men, and they worry that I don't like them. It makes them mad, it makes them sad, it makes them shout and scream. I like them. But I don't want to flirt and mother them... I don't want to flirt with you because I don't want to f**k you, and I don't want to mother you because I am not your mother."
The Duchess of Cambridge has just returned to work after having her third child, Prince Louis. She made her first post-maternity leave appearance at Sayers Croft Forest School and Wildlife Garden at Paddington Recreation Ground at start of October.
The entire essay can be found in Feminists Don't Wear Pink (And Other Lies), which is available now.