The mum-to-be was seen using her iPad to take photos of her protégé Aimee Aspinall riding Disarm during the dressage event.
Zara, who is six months pregnant, was dressed down in a long-sleeve grey top, jeans and a warm gilet by equestrian brand Musto, whose clothing line Zara helped to launch in October 2012.
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The Queen's granddaughter, who became the first royal to win an Olympic medal during London's 2012 Olympic Games, has been seen coaching Aimee on numerous occasions and is now keen to help her progress as she cannot ride herself.
The 32-year-old's appearance at the horse trials, which were held at Warwick College, came the day after she visited a maternity unit in Stroud, Gloucestershire.
Zara dressed her baby bump in a white blouse from online store ASOS, a black skirt, tights and ankle boots as she helped the ward celebrate its refurbishment and 60th birthday.