The mum-to-be took all precautions against the cold weather, wearing a woolly hat and mittens and dressing her bump in a green parka coat with a fur-lined hood.
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Warm blue and pink striped socks were also visible at the top of the royal's tan-coloured boots as she strode around the field, chatting to friends.
Olympic silver medalist Zara had three horses entered in Sunday's event.
The trio included the hotly tipped Devils Boy, Calusa Comet and Irish Bay as well as one of Princess Anne's horses named Fiddle Faddle.
Reports have suggested that 32-year-old Zara, who is due to give birth this month to her first child with husband Mike Tindall, will welcome her baby at the NHS Gloucester Royal Hospital.
The maternity unit is close to where Zara and Mike live, on Princess Anne's estate, Gatcombe Park.
The new arrival, whose gender is currently unknown, will be the Queen's fourth great-grandchild and cousin to Peter Phillips' two daughters Isla and Savannah.