A woman's grandparents who "couldn't resist" seeing her blow out her birthday candles stood outside in the garden and watched the party through the conservatory window.
Valerie and Sam Horncastle, both 89, drove more than five miles (8km) from Newmarket, Suffolk, to Kirtling, Cambridgeshire, to surprise Lauren Godfrey on her 30th birthday.
They kissed and spoke to their family through the glass on Thursday.
Mrs Godfrey said it was "bittersweet".
The mother-of-two said her grandparents were "heartbroken" that they have had to keep their distance from their family because of coronavirus.
Older people are at greater risk of becoming seriously unwell if they become infected with coronavirus, Public Health England says.
"They know it's the best thing to do but they couldn't resist coming over and see us blow out the candles," Mrs Godfrey said.
"They wouldn't even let us open a window so we just spoke through the glass."
Mrs Godfrey, who has daughters Annabella, three, and Margot, aged four months, said she had cancelled her plans for lunch, brunch and dinner because of coronavirus and decided to stay at home.
The online blogger also cancelled a charity ball that was due to have 140 guests and help raise funds for Petals, the baby loss counselling charity.
She said: "I'd been feeling a bit blue about the day but I knew the pandemic is bigger than my birthday.
"It felt like such a beautiful moment [when her grandparents turned up] but sad at the same time, it's heartbreaking really that your almost 90-year-old grandparents have to stand in the garden and watch you blow out your birthday candles."