In a post on her private Instagram account, Mrs Johnson says she feels “incredibly blessed to be pregnant again” while also revealing she suffered a miscarriage at the start of the year.
“Hoping for our rainbow baby this Christmas,” the 33-year-old began in the caption, alongside a photo of a Christmas tree bauble in the shape of a pram.
Mrs Johnson said the pregnancy loss earlier this year left her “heartbroken” but she hopes sharing the news can give comfort to others going through similar circumstances.
“At the beginning of the year, I had a miscarriage which left me heartbroken,” she wrote.
“I feel incredibly blessed to be pregnant again but I’ve also felt like a bag of nerves.
“Fertility issues can be really hard for many people, particularly when on platforms like Instagram it can look like everything is only ever going well.
“I found it a real comfort to hear from people who had also experienced loss so I hope that in some very small way sharing this might help others too.”
The couple, who wed in an intimate ceremony at Westminster Cathedral in London in May, are parents to 16-month-old son Wilfred, who was born in April 2020.
Mr Johnson, 56, is the first British prime minister to marry in office since Lord Liverpool in 1822.
He also became one of just four leaders to welcome a newborn while serving at No.10 in 150 years, following in the footsteps of David Cameron, Tony Blair and Lord Russell.
The new bub will be Carrie’s second and Boris’s seventh – the prime minister has four children with his second wife Marina Wheeler and has fathered at least one child outside of his marriages, according to Associated Press.