Sarah, Duchess of York shared new family photos as she expressed her "joy and excitement" at daughter Princess Beatrice's baby news.
The author, 64, posted previously unseen snaps of Beatrice and three-year-old daughter Sienna alongside a congratulatory message, writing: "A very proud GiGi/Mum today. Such an incredible blessing and an embrace to my heart. Wolfie, August, Ernie and Sienna will be over the moon to have the fifth member of the five aside."
One image showed little Sienna as a flower girl, wearing a white puff-sleeved dress with a sage green sash as grandmother Sarah and Beatrice, donning her daughter's floral crown, both beamed at her. The youngster had a starring role at Marissa Montgomery and Jesse Brown's wedding at Chelsea Physic Garden in London back in June.
Another photo showed Sienna, dressed in a lemon-hued ensemble and wearing a cherry-printed sun hat, as she holds her mother's hand. Beatrice, sporting sunglasses and a floral dress, is crouching down and smiling at her daughter.
A third snap featured Beatrice and her younger sister, Princess Eugenie, as they enveloped their mother, Sarah, in a group hug.
In a lengthier message over a backdrop of Beatrice and a toddling Sienna in the garden, Sarah wrote: "Darling Beatrice, words can't quite express the joy and excitement I feel as you and Edo prepare to welcome another precious addition to your beautiful family. Becoming a granny again fills my heart with so much happiness and gratitude.
"Watching you grow as a mother has been one of the greatest privileges of my life, and now to see your family blossom further is an absolute blessing. Our family is growing, and so too is the love that surrounds us. I cannot wait to meet this little one and to share in all the wonderful memories we will make together. All my love."
Sarah and her ex-husband, the Duke of York, are also grandparents to Eugenie and Jack Brooksbank's sons, August, three, and Ernest, one.
Buckingham Palace announced the happy news in a statement released on Tuesday afternoon, which read: "Her Royal Highness Princess Beatrice and Mr Edoardo Mapelli Mozzi are very pleased to announce that they are expecting their second child together in early spring; a sibling for Wolfie, aged eight, and Sienna, aged three. His Majesty The King has been informed and both families are delighted with the news."
The palace shared two family photos, one showing Edoardo embracing his beaming wife, who is wearing a black puffer jacket and a red headband, and another showing the property developer enjoying a woodland walk with his and Beatrice's daughter Sienna, and his son Christopher Woolf, known as Wolfie.
The tot will be 11th in the line of succession when he or she arrives next year.