"The prime minister and Miss Symonds are very pleased to announce their engagement and that they are expecting a baby in the early summer," a spokesperson for the couple said.
On her Instagram account, Ms Symonds said: "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. Feel incredibly blessed."
The couple have been living together in 10 Downing Street since July and have adopted a dog, Dilyn.
