The parents-to-be joined the likes of Prince Harry and Prince Charles in attending the gathering at the palace on Wednesday afternoon.
William was pictured driving his wife into the palace grounds – we're guessing one-year-old Prince George was safely strapped into a carseat in the back!
The royal couple and their infant child will be celebrating their final Christmas as a trio this year, with the arrival of a second child expected in April next year.
Kensington Palace announced Kate's second pregnancy back in September but revealed that unfortunately she was again suffering from hyperemesis gravidarum, a severe form of pregnancy sickness.
Poor Kate suffered the same condition while pregnant with Prince George and was briefly hospitalised at the start of that pregnancy.
The 32-year-old had to cancel several planned engagements earlier this year, including her first planned overseas solo engagement, in order to recuperate but lately has been looking healthy and well.
She travelled to New York with Prince William earlier this month, and yesterday made a surprise visit to a Beaver Scouts association in London, where she chatted with the youngsters and took part in a series of activities to help children learn about people living with disabilities.
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