According to the Daily Mirror Miss Symonds told friends she was five months pregnant, leading to speculation the conception occurred during the Manchester conference. The couple are also engaged. She explained on Instagram: “I wouldn’t normally post this kind of thing on here but I wanted my friends to find out from me.
“Many of you already know but for my friends that still don’t, we got engaged at the end of last year.
“And we’ve got a baby hatching early summer.”
She added: “Feel incredibly blessed.”
Mr Johnson is currently in the process of divorcing his second wife, Marina Wheeler.
Boris Johnson and Carrie Symonds are expecting a child (Image: GETTY)
The child may have been conceived at the last party conference (Image: GETTY)
The child will be the first born to a sitting Prime Minister since Florence Cameron, who was born on August 24 2010 to David and Samantha Cameron.
Mr Johnson will be the first sitting Prime Minister to marry since 1822.
Lord Liverpool, whose premiership lasted 15 years, married Mary Chester following the death of his first wife Louisa Hervey the year previously.
Like Mr Johnson Lord Liverpool was Foreign Secretary in his first Government position.
READ MORE: Boris Johnson net worth: PM to marry girlfriend Carrie Symonds
Florence Cameron was the last child born to a sitting British PM (Image: GETTY)
Miss Symonds is a former Tory communications chief.
She worked on Mr Johnson’s successful London mayoral re-election bid in 2012.