But did you notice that Kate's younger sister Pippa Middleton also made an appearance in the eighties throwback photo?
Carole, who shares three children with husband Michael Middleton, appears to be pregnant with her second child Pippa as she sports a subtle baby bump under her purple frock.
WATCH: All the times Kate and Pippa Middleton were #sistergoals
Several fans wished Kate and Carole a happy Mother's Day after Kensington Palace uploaded the photo, alongside another picture of the cake Prince George, Princess Charlotte and Prince Louis had baked for the occasion.
"Awww looks like Carole is pregnant with Pippa here," one fan noted on Instagram, while another chimed in: "Happy Mother's Day to Carole Middleton and her lovely daughters."
"What a beautiful lady Carole Middleton is. Kate looks adorable here," replied one follower, as a fourth echoed: "I have a lot of respect and love for Carole Middleton. An amazing woman who raised an amazing girl."
Carole was pregnant with Pippa at the time of this photo
Kate's choice of photo was particularly fitting as her sister Pippa, 37, has just given birth to her second child with husband James Matthews. They were already the proud parents of son Arthur, who turned two last October.
A family source told HELLO! that Pippa gave birth to a daughter on Monday 15 March at around 4:22am. The couple have decided to name their baby, who weighed 6lbs 7oz, Grace Elizabeth Jane. "Mother and baby are doing well," the source said. "She's perfect, everyone is overjoyed at such a happy arrival."
Kate's children baked a Mother's Day cake
After months of reports of Pippa's pregnancy, Carole confirmed her daughter's baby news earlier this month. In a cover interview for the April issue of Good Housekeeping, the Party Pieces founder was asked about her hopes for 2021.
"I hope we see COVID-19 coming under control so all of us can have a more normal life," said Carole. "Hopefully this year, more people will get to celebrate their special occasions with friends and family.
"I also hope to see more of my family than I could last year, including, of course, my new grandchild."
Describing herself as "hands-on" grandmother, the businesswoman also shared: "I want to run down the hills, climb the trees and go through the tunnel at the playground. As long as I am able to, that's what I'll be doing. I cook with them, I muck around dancing, we go on bike rides."
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