Zara is the daughter of Princess Anne and Captain Mark Phillips.
The couple welcomed their first daughter, Mia in January 2014.
Mike recently revealed that Queen Elizabeth held a private Christmas church service for Zara after her devastating 2016 miscarriage.
"Two years ago, (the Queen) wasn’t feeling very well and so she didn’t go to church,” he told talk show House of Rugby.
"We’d obviously been what we’d been through with our pregnancy that we lost, so we did our own little private (service) - just as a three."
Zara and Mike, who married in 2011, have had two miscarriages since their eldest daughter Mia was born.