Carrie Symonds and Boris Johnson are engaged and are expecting a baby in the summer, a spokesperson for the couple has announced.
The prime minister and Symonds were “very pleased” to make the announcement.
Former Scottish Conservatives leader Ruth Davidson was one of the first to congratulate the couple.
Retweeting the news, the MSP wrote: “Congratulations to Boris Johnson and Carrie Symonds.”
Davidson, who had been critical of Johnson during the European referendum campaign, stepped down as Scottish leader last year to spend more time with her young family.
Tom Tugendhat, chair of the Commons foreign affairs committee, called the baby and wedding announcement “lovely news”.
The Conservative MP said: “Congratulations to both Boris Johnson and Carrie Symonds. Happy news!”
The last prime minister to become a parent while in office was David Cameron, whose wife Samantha gave birth to their daughter Florence in 2010.
Tony Blair became the first sitting PM to father a child in 150 years when his wife Cherie had their son Leo in 2000.
Symonds moved into Downing Street with Johnson when he took over from Theresa May as prime minister in July.