Carrie Johnson announced her pregnancy in an emotional Instagram post to her 5,200 followers.
The 33-year-old said she was expecting another child in December as she also revealed she had suffered a miscarriage.
She shared an image of a Christmas ornament of a blue baby pram along with her touching post.
She wrote: "Hoping for our rainbow baby this Christmas.
"At the beginning of the year, I had a miscarriage which left me heartbroken.
"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 experience loss so I hope that in some small way sharing this might help others, too."
The couple welcomed their first child, Wilfred, in April 2020 - shortly after Prime Minister Boris Johnson, 57 was released from intensive care after suffering from Covid-19.
They named their son Wilfred Lawrie Nicholas Johnson, after their grandfathers and the doctors who treated Mr Johnson while he battled Covid-19.
Boris wed Carrie in a secret ceremony in Westminster in May - becoming the first PM to marry while in office for two centuries.
The ceremony at Westminster Cathedral, less than two miles from Downing Street, was the culmination of a six month "cloak and dagger" operation, insiders claim.
So secretive were the plans that even senior members of staff in Mr Johnson's office were kept in the dark about the event.
The 56-year-old PM and his 33-year-old bride were only allowed to invite 30 guests thanks to Covid regulations, having opted - unlike thousands of other couples across the UK - not to put their nuptials back.
She wore a £2,870 gown by Greek-based luxury designer Christos Costarellos.
The couple were last seen in public together at England's Euro 2020 semi-final against Denmark at Wembley Stadium earlier this month.
Wilfred was pictured enjoying beach time with mum Carrie and First Lady Dr Jill Biden during last month's G7 summit - revealing that at just 13 months old he had already inherited his father's mop of blonde hair.
