Describing how Athena looked when she was born, Beatrice wrote the newborn had been “so tiny it took more than a few weeks for the tears of relief to dry”.
She said: “I’m extremely pleased to let you know Athena is now doing really well, I have a few more answers as to what happened, but still no precise explanation.”
Beatrice expressed her hopes sharing her story would support other mothers going through the same uncertainty and that women’s health “has been left off the agenda”.
“My joys and fears in pregnancy and motherhood are the same as those experienced by millions of other women around the world,” she added.
The princess married her millionaire property tycoon husband in a private ceremony in 2020. The pair also share a three-year-old daughter named Sienna.
Mr Mapelli Mozzi has an eight-year-old son named Wolfie, from a previous relationship.
Princess Beatrice, who is ninth in line to the throne, is the eldest daughter of the Duke of York and Sarah Ferguson, Duchess of York.
She has worked with a number of charities, including the Teenage Cancer Trust and co-founding the Big Change, which helps young people to develop life skills, beyond academic subjects.