He and Symonds have already made history as the first unmarried couple to occupy the prime minister's official residence.
The former Conservative Party communications chief was first romantically linked to Mr Johnson by the media in early 2019.
Whilst this will be Symonds' first child Boris Johnson already has at least five children with his various partners over the years.
Symonds also shared the announcement - alongside an intimate photo of her and Johnson - on her Instagram account, which is set to private, according to Britain's Daily Mail.
How many children the Conservative leader has is an ongoing debate, and one that Johnson refused to answer questions on during last December's general election.
"Many of you already know but for my friends that still don't, we got engaged at the end of previous year... and we've got a baby hatching early summer", she reportedly wrote.
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"They want to hear how we're going to get Brexit done".
According to reports, the prime minister is soon to enter marriage and is expecting a child.
Mr Johnson, known for his rumpled appearance and flamboyant oratory, was previously married to Ms Marina Wheeler, and they have four children together. During the election campaign, Johnson said that he would not "put them on the pitch".
Britain announced a jump in coronavirus cases on Sunday, with 12 new infections taking the total to 35, as health minister Matt Hancock said the government was preparing for the global epidemic to get worse.
Tony Blair and his wife Cherie also had a child, a boy called Leo, whilst he was in office.
Their child will be the first child born to a sitting PM since David and Samantha Cameron's daughter Florence Rose Endellion was welcomed into the world in August 2010.
Twice-married Johnson, 55 - who split from his wife of 26 years in 2018 - has been publicly dating 31-year-old Symonds since early last year.