Boris Johnson's fiancee Carrie Symonds, who is pregnant with the couple's first child, says she has spent the past week in bed with symptoms of coronavirus.
Posting on social media, Ms Symonds said she has not yet been tested for COVID-19, the disease caused by the coronavirus.
Mr Johnson is currently self-isolating after contracting coronavirus.
Ms Symonds said on Twitter: "I've spent the past week in bed with the main symptoms of Coronavirus."
"I haven't needed to be tested and, after seven days of rest, I feel stronger and I'm on the mend."
The 32-year-old, who is at least six months pregnant, advised other expectant parents to keep up to date with relevant guidance from The Royal College of Obstetricians and Gynaecologists.
"Being pregnant with COVID-19 is obviously worrying. To other pregnant women, please do read and follow the most up to date guidance which I found to be v [sic] reassuring," she said.
Mr Johnson, 55, has already been in his Downing Street flat for seven days, the recommended amount of isolation time for those living alone.
But the prime minister will continue to remain away from others as he still is suffering from a high temperature, a symptom of COVID-19.
Ms Symonds, a former communications head for the Conservative Party, is believed to have left Downing Street more than a week ago for her flat in Camberwell in South London.
In a photo on her Instagram account, Ms Symonds can be seen lying in bed with the couple's dog, Dilyn - "Self-isolating isn't so bad with this one," the photo's caption reads.
The Chief Medical Officer placed those who are pregnant in a vulnerable group last month, which means expectant parents should reduce social contact through social distancing measures, particularly those in their third trimester.
These measures include:
The Royal College of Obstetricians and Gynaecologists say that those who are pregnant are still no more likely to contract COVID-19 than the general population.
However, the College says pregnancy in a small number of people can alter how their bodies handle severe viral infections.
The College also advises that expectant parents continue to attend antenatal and postnatal appointments as normal, unless they have had symptoms of COVID-19.
Those with symptoms should contact their community midwife to postpone routine visits until their isolation period is over.
Mr Johnson and Ms Symonds were last seen together in public at the Commonwealth Day Service on 9 March.
The couple got engaged at the end of 2019, and their first child together is due in the "early summer".
Both have been living together in residence above 10 Downing Street since the prime minister took over as leader of the Conservatives.
Mr Johnson will become the first prime minister to marry while in office in over 170 years.
Mr Johnson already has four children from his second wife.
Mr Johnson also has a daughter with art consultant Helen Macintyre and there has been speculation - about which he has refused to answer questions - that he has fathered another extra-marital child.