The actress, 41, is expecting her first baby with boyfriend Oli Green, 26. She shares daughter Marlowe with ex-fiancé, the actor Tom Sturridge.
In pictures posted on social media by the Layer Cake actor’s family and friends, Miller could be seen wearing a slip dress that showed off her bump.
Fans speculated the actor was expecting another baby girl thanks to the pink and pastel theme of the celebration.
Her house was covered in pink balloons in various shades of the colour, with the mother-to-be herself wearing a pink outfit and flower crown with blush-coloured blooms.
According to pictures, her guests were all wearing shades of pink and white.
Meanwhile, her sister Savannah added a slew of pink heart and flower emojis to her Instagram caption.
Miller and The Crown star Green made their relationship red-carpet official at a Vanity Fair party in February 2022.
Miller has yet to publicly confirm the sex of the baby - or indeed that she is pregnant.
The star showed off a baby bump in August while on holiday in Ibiza.
At the time, British Vogue was among those to confirm and congratulate Miller on her happy news.
A Tweet from the publication read: “Congratulations are also in order for former #BritishVogue cover star #SiennaMiller, who is expecting a baby with her boyfriend Oli Green. It will be a second child for the actor, who has a 10-year-old daughter, Marlowe.”
At a Vogue World party earlier in the year, she wore a sensational romantic two-piece from Schiaparelli that showed off her bump.
She was the latest celebrity to bare her baby bump on the red carpet – following in the maternity style footsteps of Rihanna who never shied from showing off her bump during her two pregnancies.
Later, Miller later changed into a red tartan set to perform onstage at London’s Theatre Royal Drury Lane, matching with some of the UK’s leading acting talent: Damian Lewis, Cush Jumbo, James Corden and James McAvoy.