She and brother-in-law Prince Harry were spotted with a group of friends as they hopped off the coach for nuptials in the Swiss mountains.
They were there for the wedding of close friend and polo player Mark Tomlinson, who married Olympic equestrian Laura Bechtolsheimer in the town of Arosa.
Dressed in a pale coat accentuated with brown fur trim, a familiar James Lock hat and a Max Mara dress she's wore previously underneath, an expectant Kate was seen walking "gingerly up the steps to the church," an onlooker tells PEOPLE. "She was being very careful in her heels."
Her husband William – in traditional tailcoat – had previously arrived due to his role as an usher at the ceremony.
As guests arrived, police cordoned off an area so locals could catch a glimpse. "There was a big crowd there, and the police closed the street," the onlooker adds.
The couple are among 250 guests, including the royals' close friends James Meade and fiancée Laura Marsham, Guy Pelly and Olivia Hunt.
The Princes often spend summer afternoons playing polo with groom Tomlinson, who attended Marlborough College with Kate. The bride was part of the London 2012 Olympics team that also included William and Harry's cousin Zara Phillips.
The family made a weekend of the trip – while William and Harry hit the slopes on Friday, Kate, who's due in July, was spotted strolling with a sled in her hands rather than ski poles.