Boris Johnson’s pregnant fiancee Carrie Symonds falls into the group of vulnerable people urged to avoid contact with those with symptoms of Covid-19.
Ms Symonds, who is expecting the couple’s baby in early summer, is also advised to take social distancing seriously.
Downing Street suggested Ms Symonds is not living with Mr Johnson at present.
Asked whether she is also living in the flat above Number 11 with Mr Johnson, the PM’s official spokesman said: “The Prime Minister of course follows all of the guidelines which have been issued by Public Health England in full.
“His circumstance is such that he will be required to self-isolate for seven days.”
Evidence shows that people can carry the virus for several days before they start showing symptoms.
But the World Health Organisation (WHO) says the main way the disease spreads is through respiratory droplets expelled by someone who is coughing.
Nevertheless, many people with Covid-19 experience only mild symptoms, especially in the early stages.
While pregnant women do not appear more likely to contract coronavirus than the general population, pregnancy itself alters the body’s immune system and response to viral infections in general.
Guidance updated on Thursday from the Royal College of Obstetricians and Gynaecologists (RCOG) says viral infections can “occasionally be related to more severe symptoms and this will be the same for Covid-19”.
It says that, while the risks are small overall, health professionals should look out for more severe symptoms of Covid-19 in pregnant women who test positive, such as pneumonia and a lack of oxygen.
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But the RCOG said the current expert opinion is that unborn babies are unlikely to be exposed to Covid-19 during pregnancy.
There is also no data at the moment suggesting an increased risk of miscarriage for pregnant women.
The RCOG reiterates Government advice that pregnant women “should pay particular attention to avoiding contact with people who are known to have Covid-19 or those who exhibit possible symptoms”.
It adds: “Women above 28 weeks’ gestation should be particularly attentive to social distancing and minimising contact with others.”
Dr Michael Head, senior research fellow in global health at the University of Southampton, said: “It is public knowledge that the Prime Minister’s partner is pregnant, and so a confirmed Covid-19 infection does give some concern around the health of mother and baby.
“It is reassuring that so far there have been few noted complications during pregnancy of infection with Covid-19.
“However, this is an emerging evidence base, so the health services will be cautious with the welfare of all expectant mothers and any associated risks.”