The pregnant royal dressed her baby bump in a white blouse from ASOS and a black skirt, accessorising her look with black tights, ankle boots and pearl earrings as she stepped out to mark the special occasion.
Mum-to-be Zara, who is due to give birth in the new year, toured the newly refurbished ward which now boasts three brand new en-suite birthing rooms, that the unit hopes will encourage more women to choose to have their babies there.
As Zara posed with the ward's brightly coloured birthday cake she stood by a plaque commemorating a visit made by her mother Princess Anne some years ago.
The Stroud Maternity Unit is not located far from Princess Anne's estate, Gatcombe Park, where Zara and her husband Mike Tindall formerly lived and where her brother Peter Phillips lives with his wife Autumn and their daughters Isla and Savannah.
Equestrian star Zara has not let her pregnancy slow her down and even carried on riding in the weeks after she announced that she was expecting her first child.
In the following months the Queen's granddaughter donned vintage clothing as she attended the Goodwood Revival Festival on 13 September and last weekend she joined her cousins Princess Beatrice, Princess Eugenie and Prince Harry for the Boodles Boxing Ball.