Eugenie, who also celebrated her 35th birthday on March 23, wrote: "Proud of you Bea bea and mighty Athena", followed by a pink-heart emoji.

Unlike her sister, Beatrice isn't active on social media so the royal wasn't tagged in the adorable post. However, her husband also appeared to beam with pride over his wife's latest project.

Taking to his own Instagram account, Edoardo Mapelli Mozzi expressed how proud he was of his "beautiful wife". Edoardo, who also shares three-year-old Sienna with Beatrice, wrote: "So much pride for my beautiful wife, who has bravely shared our story of Athena’s early arrival. Also for her support and patronage for the incredible organisation @bornecharity, which provided us with invaluable help during that challenging time [pink-heart emoji]."

Edoardo, who is also dad to eight-year-old Wolfie from a previous relationship, previously shared the birth of Athena on his Instagram account.

Back on January 29 - exactly one week after Athena's birth - Edoardo wrote: "Athena Elizabeth Rose Mapelli Mozzi. We welcomed Baby Athena into our lives last week. She is tiny and absolutely perfect. We are all (including Wolfie and Sienna) already completely besotted with her.

"Our hearts are overflowing with love for you, baby Athena [pink-heart emoji]. A massive thank you from my wife and I goes out to all the wonderful staff at the Chelsea and Westminster Hospital for their exceptional care and support during this incredibly special time."